Some Basic Concepts of Chemistry

Question
CBSEENCH11005106

Calculate the mass of sodium acetate (CH3COONa) required to make 500 mL of 0·375 molar aqueous solution. Molar mass of sodium acetate is 82·0245g mol–1.

 

Solution

1000 mL of the solution contain sodium acetate = 0·375 mole,
     therefore space space 500 space mL space of space the space solution space would space contains
sodium space acetate space equals space fraction numerator 0.375 over denominator 1000 end fraction cross times 500 space mole
space space space space space space space space space equals fraction numerator 0.375 over denominator 1000 end fraction mol
therefore space space space Mass space of space sodium space acetate space required
space space space space space equals space fraction numerator 0.375 over denominator 2 end fraction cross times space molar space mass space of space sodium space acetate
space space space space space space equals space fraction numerator 0.375 over denominator 2 end fraction cross times 82.0245 space equals space 15.380 space straight g

Question
CBSEENCH11005107

The density of 3M solution of NaCl is 1.25g ml-1.  Calculate molality of the solution.

Solution

Here comma space straight M space equals 3 space mol space straight L to the power of negative 1 end exponent
therefore space space space Mass space of space NaCl space in space 1 straight L space solution space equals space 3 space straight x space 58.5 space equals space 175.5 space straight g
therefore space space space Mass space of space 1 space straight L space solution space equals space 1000 space cross times space 1.25 space equals space 1250 space straight g
space space space Mass space of space water space in space solution space equals space 1250 minus space 175.5 space equals space 1074.5 space straight g

therefore space space space space Molality space equals fraction numerator 3 space mol over denominator 1074.5 end fraction cross times 1000 space equals space 2.79 space straight m

Question
CBSEENCH11005108

In a reaction vessel 0.184 g of NaOH is required to be added for completing the reaction. How many millilitre of 0.150 M NaOH solution should be added for this requirement?

Solution

Molar mass of NaOH = 40 g mol-1
because 
1000 mL of NaOH solution contains 0.150 mol = 0.150 x 40 = 6 g
It means that 6g sodium hydroxide is present in 1000 mL
 therefore 0.184 g sodium hydroxide would be present in 
                                          equals space 1000 over 6 cross times 0.184 space equals 184 over 6 space equals space 30.7 space mL

Question
CBSEENCH11005109

Commercially available concentrated hydrochloric acid contains 38% HCl by mass.
(i) What is the molarity of the solution (density of solution = 1·19g cm-3)?
(ii) What volume of concentrated HCl is required to make 1·0L of 0·10 M HCl?

Solution

 (i) 38% HCl by mass means that 38g of HCl is present in 100g of solution.
Volume of solution
                  equals space fraction numerator Mass space over denominator Density end fraction space equals space fraction numerator 100 over denominator 1.19 end fraction space equals 84.03 space cm cubed
Moles of HCl = fraction numerator 38 over denominator 36.5 end fraction space equals space 1.04
Molarity = fraction numerator No. space of space moles space of space HCl over denominator Vol. space of space solution space left parenthesis in space cm cubed right parenthesis end fraction cross times 100
             equals space fraction numerator 1.04 over denominator 84.03 end fraction cross times 1000 space equals space 12.38 space straight M
(ii) By applying normality equation,
                       straight M subscript 1 straight V subscript 1 space equals space straight M subscript 2 straight V subscript 2 comma space we space have
12.38 space straight M space cross times space straight V subscript 1 space equals space 0.10 space cross times space 1.0 space straight L
therefore space space space space space straight V subscript 1 space equals space fraction numerator 0.10 space cross times space 1.0 over denominator 12.38 end fraction
space space space space space space space space space space space space space equals space 0.00808 straight L
space space space space space space space space space space space space space equals space 8.08 space cm cubed