Science Chapter 3 Atoms And Molecules

We have given,

Mass of boron = 0.096g

Mass of oxygen = 0.144g

Mass of sample =0.24g

Thus, percentage of boron by weight in the compound = $\frac{0.096 \mathrm{x} 100}{0.24} =40\%$

Thus, percentage of oxygen by weight in the compound =$\frac{0.144 \mathrm{x} 100}{0.24} =60\%$

When 3.0 g carbon is burnt in 8.00 g oxygen, 11.00 g carbon dioxide is formed. It means all of carbon and oxygen are used up and carbon and oxygen are combined in the ratio of 3 : 8 to form carbon dioxide. Thus when there is 3 g carbon and 50.0 g oxygen, then also only 8 g oxygen will be used and 11.0 g carbon dioxide will be formed. The remaining oxygen is not used us. This indicates law of definite proportions which says that in compounds, the combining elements are present in definite proportions by mass.

A group of atoms carrying a charge is known as a
polyatomic ion. For example,  hydroxide ion (OH^–), carbonate ion (CO₃^–2).

Chemical formulae of the following is:

(a) Magnesium chloride – MgCl₂

(b) Calcium oxide – CaO

(c) Copper nitrate – Cu(NO₃)₂

(d) Aluminium chloride – AlCl₃

(e) Calcium carbonate – CaCO₃

(a) Quick lime(CaO) – Calcium and oxygen

(b) Hydrogen bromide(HBr) – Hydrogen and bromine

(c) Baking powder(NaHCO₃) – Sodium, Hydrogen, Carbon and Oxygen

(d) Potassium sulphate (K₂SO₄)– Potassium, Sulphur and Oxygen

(a) Ethylene, C₂H₂ = 2 x 12 + 2 x 1 = 26 u

(b) Sulphur molecule, S₈ = 8 x 32 = 256 u

(c) Phosphorus molecule, P₄ = 4 x 31 = 124 u

(d) Hydrochloric acid, HCl = 1 x 1 + 1 x 35.5 = 36.5 u

(a) 1 mole of nitrogen atoms = 14 u = 14 gm

(b) 4 moles of aluminium atoms = 4 x 27 = 108 u = 108 gm

(c) 1 mole of sodium sulphite, Na₂ SO₃ = (2 x 23 + 1 x 32 + 3 x 16) = 126 u

10 moles of sodium sulphite = 126 x 10 = 1260 u = 1260 gm.

(a) Formula of oxygen is O₂.
1 mole of oxygen gas = 32 gm
 12 gm of oxygen gas = 12/32 = 0.375 mole

(b) Formula of water is H₂O. 
 18 gm of water = 1 mole
 20 gm of water =  20/18 = 1.1 mole

(c) Formula of carbon dioxide CO₂.
44 gm of carbon dioxide = 1 mole
22 gm of carbon dioxide = 0.5 mole

(a) 1 mole of oxygen atoms = 16 gm

0.2 moles of oxygen atoms = 16 x 0.2 = 3.2 gm

(b) 1 mole of water molecules = 18 gm
0.5 mole of water molecules = 0.5 x 18 =9.0 gm

1 mole of aluminium oxide, Al₂O₃ = 2 X 27+3 X 16 = 102 u = 102 gm

So,       
102 gm has Al₂O₃ has = 6.023 X 10²³ Al₂O₃ molecules

             0.56 gm Al₂O₃ has  6.023 X1023 X 0.56
                                               102
         
         = 3.01 X 10²¹ Al₂O₃ molecules
     
1 molecules of Al₂O₃ gives =2Al³⁺ ions

Hence 0.56 gm Al₂O₃ gives = 2 X 3.01 X 10²¹ Al³⁺ ions

  = 6.023 X 10²¹ aluminium ions

Law of consveration of mass state that mass can neither be created nor be destroy in chemical reaction. For example, take a solution of lead nitrate in water. Weigh it. Let it be x gm. Then add a given amount of solution (y gm) of sodium chloride. It will be found that a white precipitate is formed. Weigh the total solution. It will be found that total weight of the contents is (x + y) gm, i.e., equal to the total weight of the two solutions taken. Hence, it can be inferred that mass is neither created nor lost even during a chemical change. Similar results are obtained when any other chemical reaction has taken place. Only care to be taken is that no material is allowed to escape during the reaction.

50 g of 10% lead nitrate means the solution contains 5 g lead nitrate and 45 g water. Similarly, 50 g of 10% sodium chloride means the solution contains 5 g sodium chloride and 45 g water.
Thus total content before reaction = 5+5+90 = 100 g
After reaction, amount of water = 90 g
Amount of precipitate = 6.83 g
Since according to law of conservation of mass, the total mass of reaction mixture = 100 g
So, Amount of sodium nitrate = 100 - 90 - 6.83
                                       = 3.17 g

(i) Law of constant proportion. According to law of constant composition or proportion, all pure samples of a compound contain the same elements combined together in the same proportion by mass. For example, a sample of water would always contain hydrogen and oxygen in the ratio of 1 : 8 by mass irrespective of the source of water. If we decompose 100 grams of pure water by passing electricity through it then we get 11 grams of hydrogen and 89 grams of oxygen showing that hydrogen and oxygen are combined together in water in the same constant proportion of 1 : 8 by weight.

(ii) Law of multiple proportions. According to law of multiple proportions, when two elements combine to form two or more compounds, then the weights of one of these elements which combine with a fixed weight of the other element, bear a simple ratio to one another. For example, hydrogen and oxygen combine to form water (H₂O) and hydrogen peroxide (H₂O₂) under different conditions. In water 2 grams of hydrogen combine with 16 grams of oxygen while in hydrogen peroxide, 2 grams of hydrogen combine with 32 grams of oxygen. Now the weights of oxygen which combine with a fixed weight of hydrogen (2 grams) in water and hydrogen peroxide respectively are 16 and 32 grams which are in simple ratio of 16 : 32 or 1 : 2.

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.Each substances is made by extremely small particles called atoms.

The postulates of this theory may be stated as follows:
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.

Explanation on the basis of Dalton’s atomic theory:
According to Dalton’s atomic theory, each element consists of atoms which are similar and have same weights. Further atoms of one element combine with atoms of another to form compounds. for example that x atoms of element A combine with y atoms of element B and the compound formed is AxBy (Dalton’s atomic theory).
If ‘a’ stands for the atomic mass of A and ‘b’ for that of B,then

Now a and b (atomic masses of elements) are fixed, x and y are also fixed whole numbers according to atomic theory. Therefore per cent of A and B in the compound is also constant. This shows that the composition of various elements in a compound is also fixed. This is the law of constant proportion.

In water 1 g of hydrogen combines with 8 g of oxygen. And in hydrogen peroxide, 2 g of hydrogen combines with 16 g of oxygen. The ratio of oxygen in water and hydrogen peroxide combining with the same amount of hydrogen i.e., 1 g is 8 : 16 or 1 : 2—a small integral ratio. This is law of multiple proportions which defines as under.

When two elements combine to form more than one compound, the masses of one element that combine with a given mass of the other element in different compounds are in the ratio of small whole numbers.

Some of the drawbacks of Dalton’s atomic theory of matter are:

1. According to Dalton’s atomic theory, matter is indivisible, i.e., which cannot be divided. But now it is known that under special circumstances, atoms can be divided into still smaller particles called electrons, protons and neutrons.

2. Dalton’s atomic theory fails to explain why substances like charcoal, graphite and diamond have different properties when all these substances are made up of the same type of atoms, called carbon atoms.

3. Dalton’s atomic theory postulated that all the atoms of the same element have exactly the same mass. It is now known that isotopes are atoms of the same element but have different masses.

4. It is now known that some atoms called isobars have same masses but these belong to different elements. Whereas Dalton’s atomic theory says that atoms of different atoms have different mass.

According to Dalton’s atomic theory, matter is indivisible, i.e., which cannot be divided. But now it is known that under special circumstances, atoms can be divided into still smaller particles called electrons, protons and neutrons. This was the major drawback of Dalton's atomic theory of matter.

The ratio of hydrogen and nitrogen in the first sample of ammonia is 9:42 or 3:14.
According to ‘law of definite proportions' the second sample of ammonia should also contain hydrogen and nitrogen in the ratio 3:14.

The above experiment illustrates law of multiple proportions. 16 g and 32 g of oxygen respectively are required to combine with a fixed weight, i.e., 12 g of carbon to form carbon monoxide and carbon dioxide. The ratio of oxygen which combines with a fixed weight of carbon is 16 : 32 or 1 : 2, a simple whole number. This is law of multiple proportions.

Mass of reactants = Mass of sodium carbonate + Mass of ethanoic acid
                       = 5.3+6.0 = 11.3 g
Mass of products = Mass of carbon dioxide + Mass of water + Mass of sodium ethanoate
                      = 2.2 + 0.9 +8.2 = 11.3 g
Since the mass of products is equal to mass of reactants. Thus mass is neither creater nor lost during the given chemical change or the observation made is in agreement with the law of conservation of mass.

We know that, 1 g of hydrogen reacts with oxygen =8g

Therefore, 3g of hydrogen reacts with oxygen =8x 3 =24g

Thus, 24g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

Atoms are indivisible particles, which cannot be created nor destroyed in a chemical reaction. This is law which result of the law of conservation of mass.

The relative number and kinds of atoms are constant in a given compound. This theory explain the law of definite proportions.

The smallest particle that retains properties of an element. Atom is Composed of electrons and a nucleus.

Atoms of a solid exist in closely packed and compressed to each other. A solid element is a cluster of atoms. The properties of an element in the solid state is not of single atom of that element but due to cluster of atoms. For example, diamond and graphite though both are composed of carbon atoms but due to different arrangements of carbon atoms in these, they differ in their physical and chemical properties.

Atoms are very small atoms, they are smaller than anything that we can imagine or compare with. More than millions of atoms when stacked would make a layer barely as thichk as a sheet of paper. Nowdays, It is possible to take in depth photographs of the surfaces of the elements. The surface images of silicon with a scanning tunneling microscope (STM) shows that the silicon atoms are arranged in a regular pattern.

In the beginning, the names of elements were derived from the name of the place they were found for the first time or from some specific colours. For example, the name of copper was taken from Cyprus, a place where it was found. Gold was taken from the English word meaning yellow.

Many of the symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (upper case) and the second letter as a small letter (lower case). For example, Hydrogen is written as H and Aluminium is written as Al.

Some symbols are formed from the first letter of the name and a letter, appearing later in the name. Examples : Chlorine, Cl and Zinc, Zn.

Some other symbols are taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from Latin name ferrum.

In order to write the chemical reactions conveniently, each element is represented by a ‘letter’ or a group of two letters called symbol. Thus hydrogen is represented as ‘H’. Calcium is represented as ‘Ca’.

Symbol of Common Elements

Atomic mass of hydrogen =1u,
Atomic mass of oxygen =16u
SO the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is
= 2 x1+1 x16 = 18u

The molecular mass of HNO₃ = the atomic mass of H+ the atomic mass of N +3 x the atomic mass of O =1+14+48 =63u

i) Copper (Cu)
ii)Calcium (Ca)
iii)Oxygen (O)
iv)Magnesium (Mg)
v)Zinc (Zn).

Silver (Ag), Gold (Au), Lead (Pb), Iron (Fe), Sodium (Na).

Mercury (Hg),
Potassium (K),
Silver (Ag),
Boron (B),
Chlorine (Cl),
Gold (Au),
Lead (Pb),
Copper (Cu),
Aluminium (Al).

Dalton’s atomic theory states that each atom has a characteristic mass and all atoms of an element are identical. Since, the mass of an atom is very very small, it was a difficult task to determine the mass of an individual atom. Therefore, it was decided to measure the atomic mass of an atom. Atomic mass is the relative atomic mass of an atom as compoared to some particular atom. For example, carbon and oxygen combine to form carbon monoxide. It is observed that 3 g of carbon combines with 4 g of oxygen. In other words.
carbon is 4/3 times its mass of oxygen, since,  oxygen combines with a large number of elements, oxygen atom was taken as standard and 1/16 of mass of oxygen was taken as one atomic mass unit (u).
Now a days 1/12 of the mass of C-12 isotope of carbon is taken as the standard reference for measuring atomic masses.

The atomic mass unit (abbreviated as amu) is unit that is used to express the atomic masses of atoms and molecule at massed of compounds. One atomic mass unit is a mass unit equal to exactly one twelfth (1/12^th) the mass of one atom of carbon-12. The atomic masses of all elements have been found relative to an atom is basically the one twelfth the mass of one atom of carbon-12 isotope.

1/12^th of carbon atom is basically the one twelfth the mass of one atom of carbon -12 isotope.

Individual atoms of most elements do not exist independently. These either aggregate to form elements or form molecules or ions which also aggregate in large numbers to form the matter that is available to us.

Atomic masses of some common elements

The size of an atom is very very small. Further atoms of most elements do not exist independently. So it is impossible to see an atom with naked eyes.

A molecule is the smallest-particle of an element or of a compound which is stable under ordinary conditions and can exist freely and shows all the properties of that element or compound.
A molecule may be made up of one, two or more atoms. Molecules with only one atom are called monoatomic, e.g., helium, argon, neon.
Molecules with two atoms are diatomic. These may have similar or different atoms, e.g., chlorine (Cl₂), oxygen (O₂), hydrochloric acid gas (HCl). Similarly, there are molecules containing three atoms (H₂O, CO₂), four atoms (P₄, NH₃) and so on.
Arrangement of atoms in molecules:
The atoms in a molecule are chemically bonded in a definite pattern. This arrangement of atoms is given by the structural formula of the compound. For example, structural formula for water is H—O—H. This is a simple illustration for the structure of water. In fact water and all other molecules exhibit definite spatial arrangements depending on the nature of chemical bonding between atoms.
Fig. 3.3  shows molecular model for water, ammonia and sulphur.

A molecule is usually formed when at least two atoms of the same or different kinds combine. If two or three atoms of the same kind of element combine, then it is referred to as molecule of an element or simply molecule. For example, molecule of oxygen (O₂) is formed by the combination of two oxygen atoms. Molecule formed by the union of two or more atoms of different elements is called a molecule of a compound or simply compound. For example, a molecule or a compound of carbon dioxide is formed by the union of one carbon atom and two oxygen atoms. However, the noble gases consists of molecules of single atoms only. For example, helium exists as He.

It is true that a molecule is the smallest particle of a substance which has the properties of that substance and is stable. In case of gases as example oxygen, a molecule (O₂) is stable and gives properties of oxygen gas, whereas atom ‘O’ is not stable and does not represent oxygen gas. In case of noble gases such as helium, a molecule is of monoatomic helium atom (He) and represents helium gas and is also stable. In case of a compound, e.g., carbon dioxide, properties of carbon dioxide molecule are the properties of carbon dioxide gas. A molecule of carbon dioxide is stable.

(ii) Argon molecule and (v) Neon molecule are monoatomic molecules. Here molecule is formed of only single atom.

(i) Chlorine molecule, (iv) Nitrogen molecule and (vii) Hydrogen molecule are diatomic molecules. These are formed by the union of two atoms of the same element.

(iii) Hydrogen chloride and (vi) Sulphur dioxide are molecular compounds and are formed by the union of different kinds of atoms.

The number of atoms combined together to form a stable molecule of an element is called its atomicity.

For example helium (He), neon (Ne) is a monoatomic and its atomicity is 1. In same manner  oxygen (O₂) diatomic, therefore its atomicity is 2.

Since all matters are composed of tiny particles called atoms or molecules. Therefore, the basic unit of all material substance is atom and molecule. 

Molecules containing more than four atoms are called polyatomic molecules. Examples are : sulphur (S₈), ethyl alcohol (C₂H₅OH), sugar (C₁₂H₂₂O₁₁), a form of carbon called buckminsterfullerene (C₆₀).

The properties of matter depend upon the properties of atoms or molecules of which they are composed.

The (ii) statement that two molecules of hydrogen combine with one molecule of oxygen to give two molecules of water is correct because oxygen and hydrogen exist as molecules and not as atoms.

2Cl indicates 2 atoms of chlorine and Cl₂ indicates one molecule of chlorine.

Helium (He) and Neon (Ne) are monoatomic molecules.

Chlorine (Cl₂) and Oxygen (O₂) are diatomic molecules.

Water (H₂O) and carbon dioxide (CO₂) are triatomic molecules.

Ammonia (NH₃) and hydrogen peroxide (H₂O₂) are tetra-atomic molecules.

Metals exist as atomic crystals and do not form molecules. Generally, metals and other elements like carbon and silicon do not have single molecular structure but consist of a very large indefinite number of atoms bonded together. A form of carbon bonded with sixty atoms is called buckminsterfullerence.

Non-metals exist as molecules containing one or more atoms. For example, argon exists as Ar and nitrogen as N₂.

Compounds containing non-metals only exist as molecules. For example, compound of nitrogen and hydrogen is a ammonia molecule. Compound of carbon and oxygen is a carbon dioxide molecule.

Compounds containing metal and non-metal exist as ions. For example, compound of sodium and chlorine consists of positively charged sodium ions and negatively charged chloride ions.

It can defined by the bonding properties and valency of metal and non-metal.
(a) A is a non-metal.
(b) A is a metal or non-metal like carbon or silicon.
(c) B is a non-metal.
(d) A and B, both are non-metals.
(e) A is a metal and B is a non-metal.
(f) A and B are non-metals.
(g) A is a metal and B is a non-metal.
(h) B is a non-metal.

The number of atoms constituting a molecule is knowns as its atomicity. For example in hydrogen molecule ( H₂ ), its atomicity is 2 becuase it is made of 2 hydrogen atom. 

An atom is the smallest particle of an element which may or may not have independent existence. On the other hand, ‘molecule’ is the smallest particle of the element or compound which is capable of independent existence. For example, helium (He) is an atom and can exist as such, whereas hydrogen atom (H) cannot exist as such but exists as a molecule, i.e., H₂. A molecule may be made up of similar atoms (homo atomic molecule) or dissimilar atoms (hetero atomic molecule such as HCl).

Copper carbonate is a hetero atomic molecule. It contains three types of atoms, i.e., one atom of copper, one atom of carbon and three atoms of oxygen.

Atoms : Argon, Sodium, Neon.
Molecules : Oxygen, Nitrogen, Chlorine.

When water molecule decomposes it gives hydrogen and oxygen. according to law of conversation of mass " mass neither be created nor be destroy but change into one form to other form. Thus, 2 molecules of water gives 2 molecules of hydrogen, i.e., 4 atoms of hydrogen and one molecule of oxygen, i.e., two atoms of oxygen. Hence, one molecule of water on decomposition gives 2 atoms of hydrogen and one atom of oxygen.

(i) An atom of hydrogen is the smallest particle of hydrogen element which does not have independent existence. A molecule of hydrogen has two hydrogen atoms bonded together and is the smallest particle of hydrogen element which has independent existence.

(ii) A molecule of oxygen has two atoms of oxygen only. It is the smallest particle of oxygen element which can exist independently. A molecule of water is a compound obtained from two hydrogen atoms and one oxygen atom.

An ion is a charged particle. It can be either positively charged or negatively charged. A negatively charged ion is called an anion and a positively charged ion is called a cation. Ions may consist a single charged atom or a group of atoms that have a net-charge on them. Such a group of atoms carrying a net charge is known as a polyatomic ion. For example, Na⁺ is a single charged atom and is known as sodium cation. Cl^– is a single charged atom and is known as chloride anion. NO^-₃ is a group of atoms carrying net negative charge and is known polyatomic anion.

An ionic compound is a compound formed by ions bonding together through electrostatic forces. Such compounds are formed from combination of metals with non-metals. For example, sodium chloride is an ionic compound. Its constituent particles are positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl^–).

Sodium fluoride — Sodium cation and fluoride anion

Potassium bromide — Potassium cation and bromide anion.

Calcium oxide — Calcium cation and oxide anion

Silver sulphide — Silver cation and sulphide anion

Chemical formula: It is an expression which states the number and type of atoms present in a molecule of a substance. For example, there are 6C atoms and 14 H atoms in a hexane molecule, which has molecular formula C₆H₁₄.

Valency define as,  the combining capacity of an element. It can be used to find out how many atoms of an element will combine with the other element to form a chemical formula. For example, hydrogen has a valency of +1 and chlorine has a valency of –1, so one atom of hydrogen combines with one atom of chlorine to form hydrochloric acid. Further oxygen has a valency of –2, so one atom of oxygen combines with 2 atoms of hydrogen to form water molecule, H₂O.

All elements do not form ions so their valencies do not have a charge. For example, carbon and silicon have a valency of 4 and nitrogen has a valency of 3. Further molecules containing only non-metals are formed without a charged valency. For example, in carbon tetrachloride (CCl₄), carbon has a valency of 4 and chlorine has a valency of 1.

In order to write formula of a compound comprised of cations and anions, the valencies of the ion must be known. Let us consider a compound composed of cation A with valency x⁺ and anion B with valency y^– . Then the following steps may be followed to write a molecular formula.

(i) Write down the symbols of the cation and anion side by side.

A B

(ii) Write their valencies at top corners as

A^x+ B^y–

(iii) Interchange between the ions their valencies and these are placed on the lower side of each radical or used as subscripts.

(iv) If a radical is multi-atomic, use a small bracket around it.

(v) Eliminate the common factor, if any, from the numbers used in subscripts.

vi) The valencies or charges on the ion must balance.

vii) When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.

viii) In compounds formed with polyatomic
ions, the ion is enclosed in a bracket
before writing the number to indicate the
ratio. Thus calcium phosphate, compound of calcium ion (valency 2⁺) and phosphate ion (valency 3^–) is written as Ca₃(PO₄)₂.

The same method is used to write formula even for non-ionic substances.

(a) Zinc sulphate                                      (b) Ammonium carbonate

Calcium has a valency of +2. There are two Ca atoms. Thus Ca carries a total of +4 charges. Therefore, pyrophosphate has a valency of -4. Since ferric ion has a valency of +3, the formula of ferric pyrophosphate is Fe₄(P₂O₇)₃.

Aluminium chloride = AlCl₃

Aluminium sulphate = Al₂(SO₄)₃

Sodium chloride = NaCl

Sodium sulphate = Na₂SO₄

In a ionic compound the charge excange, thus Valency of Z = –3

Valency of Al = +3

Formula of aluminium salt of Z = AlZ.

Valency of SO₄^–2 is –2 and that of metal is +3 [since it combines with 3 chloride ions (–1)].

∴
Metal sulphate = M₂(SO₄)₃.

Molecular formula of a substance is the symbolic representation of its molecule. It gives the number and kind of atoms united chemically. For example, water is given by H₂O. H₂SO₄ represents one molecule of sulphuric acid.

Significance of a formula 

To signify the information derived from a formula let us consider carbon dioxide whose formula is CO₂. It indicates :

(i) The elements present in carbon dioxide are ‘carbon’ and ‘oxygen’.

(ii) One molecule of carbon dioxide is given by CO₂.

(iii) On molecule of carbon dioxide has one atom of carbon and two atoms of oxygen.

(iv) In carbon dioxide, carbon and oxygen are in a weight ratio of 12 : 32.

(v) Mass of one molecule (relative weight or molecular weight) of carbon dioxide is 44 u, 12 u contributed from carbon atom and 32, from the two oxygen atoms.

(vi) It shows that the valency of oxygen here is 2 and that of carbon is 4.

(vii) Molecule of CO₂ is not charged and is not an ion.

The chemical formula of calcium oxide is CaO because writing formula of a compound, valencies are divided by highest common factor. Similarly formula for aluminium nitrate is AlN and not Al₃N₃.

(i) Aluminium oxide — Al₂O₃

(ii) Aluminium chloride — AlCl₃

(iii) Hydrogen sulphide — H₂S

(iv) Calcium hydroxide — Ca(OH)₂

(v) Ammonium sulphate — (NH₄)₂SO₄

(vi) Calcium bicarbonate — Ca(HCO₃)₂

(vii) Ammonium phosphate — (NH₄)₃PO₄

(viii) Silver nitrate — AgNO₃

i) Al₂(SO₄)₃ — Aluminium sulphate

(ii) MgCl₂ — Magnesium chloride

(iii) K₂SO₄ — Potassium sulphate

(iv) KNO₃ — Potassium nitrate

(v) CaCO₃ — Calcium carbonate

(vi) (NH₄)₂CO₃ — Ammonium carbonate

(vii) H₂CO₃ — Hydrogen carbonate

(viii) SO₂ — Sulphur dioxide

The formulae of given compounds are:

(i) sodium oxide — Na₂O

(ii) aluminium chloride — AlCl₃

(iii) sodium sulphate — Na₂SO₄

(iv) magnesium hydroxide — Mg(OH)₂

(i) Al₂(SO₄)₃ —Aluminium sulphate

(ii) CaCl₂ — Calcium chloride

(iii) K₂SO₄ — Potassium sulphate

(iv) KNO₃ — Potassium nitrate

(v) CaCO₃ — Calcium carbonate.

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of substance. it is therefore the relative mass of a molecule expressed in atomic mass units.

The formula mass of a substance is the sum of atomic masses of all atoms in a formula unit of a compound. Formula mass is used for substances whose constituent particles are ions. In ionic compound, one molecule may contain several positive and negative ions. One molecule of sodium chloride contains six Na⁺ and 6 C1^– ions. But formula unit is NaC1. So formula mass is used in ionic compounds.

Molecular mass of given compounds :

A mole signifies a collection of 6.023 x 10²³ atoms, molecules, ions, etc. Thus, 1 mole of oxygen atom refers to 6.023 x 10²³ atoms of oxygen.
A mole also refers to the mass in grams numerically equal to the molecular mass or atomic mass in atomic mass units. Thus, mole signifies both number and amount in grams. For example,
One mole of hydrogen molecule refers to

(i) 6.023²³ x 10²³ molecules of hydrogen.

(ii) 2.016 g (equivalent to molecular mass in u i.e., 2.016 u) of hydrogen.

Utility of mole concept.

(i) From the number of moles of a substance, we can calculate the number of elementary particles because the number of moles of a substance is directly proportional to the number of elementary particles.

(ii) One mole of a gas occupies 22.4 litres at S.T.P. (273 K and 1 atm).

(iii) One mole of any gas under the same conditions of temperature and pressure occupies the same volume.

(iv) One mole is equal to molecular mass in grams which is equal to 6.023 x 10²³ atoms, molecules, ions etc. Thus, we can calculate absolute masses of atoms and molecules.

The number 6.023 x 10²³ is referred to as Avogadro number constant and is denoted by symbol N_A.

Gram atom or Gram atomic mass. The atomic mass of an element expressed in grams is called gram atomic mass or gram atom.

1 gram atom of any element contains 6.023 x 10²³ (Avogadro number) atoms of the

element. It is equal to 1 mole of atoms.

1 gram atomic mass = 6.023 x 10²³ atoms = 1 mole.

(Avogadro number)

The gram atomic mass of an element is equal to the masses of its 6.023 x 10²³ atoms. Examples. One gram atom of hydrogen atom weighs one gram and contains 6.023 x 10²³ hydrogen atoms. 1 mole of hydrogen atom also weighs 1 gram.

The molecular mass of a substance (compound) expressed in grams is called its gram molecular mass. Ionic compounds do not contain molecules (they consist of ions) and therefore, molecular masses of ionic compounds are equal to their formula masses.
A gram molecular mass of a compound is equal to the mass of 6.023 x 10²³ molecules or 1 mole of the substance.
Examples;
(i) A gram molecular mass of hydrogen molecules (molecular mass = 2)
weighs 2 grams and contains 6.023 x 10²³ hydrogen molecules.

(ii) A gram molecular mass of sodium chloride (molecular or formula mass = 58.5) weighs 58.5 grams and contains 6.023 x 10²³ sodium chloride units.

Atomic mass of any substance indicates the times it is heavier than 1/12th of C-12 atom. Now atomic mass of H atom is one u, it means that one C-12 atom is 12 times as heavy as one H atom or to say 1000 C-12 atoms are 12 times as heavy as 1000 H atoms or 6.023 x 10²³ C-12 atoms (12 grams C-12 are found to contain 6.023 x 10²³ atoms) are 12 times as heavy as 6.023 x 10²³ H atoms. Since atomic mass of C-12 is 12 u and that of H atom is 1 u, it can be inferred that weight equal to atomic mass of C-12 or H atom contains the same number of atoms. Thus, it can be shown that gram atomic mass or gram molecular mass of all substances have the same number, i.e., 6.023 x 10²³ particles.

Mole:  Mole is the measurement in chemistry. It is used to express the amount of a chemical substance.
One mole is defined as the amount of substance of a system which contains as many entities like, atoms, molecules, and ions as there are atoms in 12 grams of carbon - 12'.
Avogadro number:  The number of the particles present in one mole of any substance is equal to 6.022x10²³. This is called Avogadro's number or Avogadro's constant. 
1 mole of sodium (6.023 x 10²³ atoms) weighs 23 g (atomic mass). And 1 mole of oxygen (6.023 x 10²³ molecules) weighs 32 g (molecular mass). Thus, particles present in 1 mole of oxygen and sodium is same such as (6.023 x 10²³ atoms) but the weight could not be same.

The metal and non-metal contain charged species. The charged species are known as ion. It can be negtive or positive. A negatively charged ion is called anion and the postively charged ion is called cation. 

(i) 2 moles of chlorine atoms in one mole of Cl₂O₇

(ii) 1 mole of chlorine atoms in one mole of HCl

(iii) 2 moles of chlorine atoms in one mole of BaCl₂

(iv) 3 moles of chlorine atoms in one mole of AlCl₃

(v) 3 moles of chlorine atoms in one mole of FeCl₃

(vi) 4 moles of chlorine atoms in one mole of CCl₄.

Atom of C-12. 

Molecular formula gives information of the number and kind of atoms in each molecule of a substance.

Avogadro Number.

These are isotopes of each other.

2.016 gram.

Mass = molar mass  x number of moles
m= M x n =16 x0.5 = 8 g

Tips: -

16 gram.

Molecular weight of carbon dioxide is 44g/mol.

Such that CO₂ contains 1 carbon and 2 oxygen atom. Thus,

1 x12 +2x16 =44  (mass of carbon =12, mass of oxygen =16)

Now by dividing the given grams (11g) by the molecular weight of CO₂:

11/44 =0.25 mol

Four moles of oxygen atoms are present in 1 mole of KMnO₄.

6.022 x10²² atom of Helium weight 4g
Therefore 6.022 x 10²² atoms of Helium will weighs

4 x6.022 x10²²/6.022 x10²³ =0.4g

Total molecular mass of water =18 g
mass of oxygen is  =16g
Thus, the percentage of oxygen in water is,
16/18 x100 =88.9%

The above statement conforms to Law of Constant Proportion.

Law of multiple proportions.

Atom is smallest particles of an element. 

Molecule is smallest particle of compound.

Carbon-12 atom.