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CBSE chemistry

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Question
CBSEENCH12010036
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Out of BaCl2 and KCl, which one is more effective in causing coagulation of a negatively charged colloidal Sol? Give reason.

Solution

According to the Hardy–Schulze rule, the greater the valency of a flocculating ion, the greater will its coagulating power. In Between Ba2+ (from BaCl2) and K+ (from KCl), Ba2+ has greater valency 2 whereas KCl has valency 1. Therefore, BaCl2 will be more effective in causing the coagulation of a negatively charged colloidal sol.

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Question
CBSEENCH12010037
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What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 1/3rd of tetrahedral voids?

Solution

Number of tetrahedral voids formed = 2 x Number of atoms of element Y

Number of atoms of element Y in the ccp unit cell = 4

Number of tetrahedral voids formed = 2 x 4 = 8


Number of tetrahedral voids occupied by atoms of X = 1/3x8

Ratio of the numbers of atoms of X and Y = 8/3: 4 =2: 3


Hence, the formula of the compound is X2Y3.

Question
CBSEENCH12010038
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What is the basicity of H3PO4?

Solution

The basicity is defined as the number of hydrogen atom replaceable by a base in a particular acid.  H3PO4 has three ionizable hydrogen atoms.

 Hence, its basicity is 3.

Question
CBSEENCH12010041
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 What is meant by positive deviations from Raoult's law? Give an example. What is the sign of mixH for positive deviation?

OR

Define azeotropes. What type of azeotrope is formed by positive deviation from Raoult's law? Give an example.

Solution

The solutions that do not obey Raoult’s law over the entire range of concentration are known as non-ideal solutions. They have vapour pressures either higher or lower than those predicted by Raoult’s law. If the vapour pressure is higher, then the solution is said to exhibit a positive deviation from Raoult’s law.

 A mixture of ethanol and acetone is an example of a solution that shows a positive deviation from Raoult’s law.

 In case of solutions showing positive deviations, absorption of heat takes place that  mixH has a positive sign.

 OR

Azeotropes are the binary mixtures of solutions that have the same composition in liquid and vapour phases and that have constant boiling points.

 A minimum-boiling azeotrope is formed by solutions showing a large positive deviation from Raoult’s law at a specific composition.

 Example: An ethanol–water mixture containing approximately 95% ethanol by volume

Question
CBSEENCH12010042
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Following reactions occur at cathode during the electrolysis of aqueous silver chloride solution:


Ag+ (aq) + e-  →  Ag(s)          E° = +0.80 V

 H+ (aq) + e-   → 1/2 H2 (g)     E° = 0.00 V

 On the basis of their standard reduction electrode potential (E°) values, which reaction is feasible at the cathode and why?

Solution

We have given:

 Ag+ (aq) + e- → Ag(s)          E° = +0.80 V

 H+ (aq) + e-→ 1/2 H2 (g)      E° = 0.00 V

 

The relationship between the standard free energy change and emf of a cell reaction is given by 
∆ G = – nFE(cell)

Thus, the more positive the standard reduction potential of a reaction, the more negative is the standard free energy change associated with the process and, consequently, the higher is the feasibility of the reaction.
Since E 0 Ag+/Ag has a greater positive value than E0 H+ /H, the reaction which is feasible at the cathode is given by

 Ag+ (aq) + e- → Ag(s)         

 

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