JEE Advance - Chemistry (1978 - No. 1)

What is the molarity and molality of a 13% solution (by weight) of sulphuric acid with a density of 1.02 g/ml? To what volume should 100 ml of this acid be diluted in order to prepare a 1.5 N solution?
1.35 M, 1.52 m, 180 ml
1.20 M, 1.40 m, 170 ml
1.40 M, 1.60 m, 190 ml
1.30 M, 1.50 m, 175 ml
1.45 M, 1.65 m, 185 ml

Explanation

1st Part : Volume of 100 g of 13% H2SO4 solution = $${{100} \over {1.02}}$$ mL = 98.04 mL

13 g of H2SO4 $$\equiv$$ $${{13} \over {98}}$$ $$\equiv$$ 0.1326 mol of H2SO4

$$\therefore$$ 98.04 mL solution contains 0.1326 mol of H2SO4.

Hence, molarity of the solution = $${{0.1326} \over {98.04}} \times 1000$$ = 1.35 mol L$$-$$1

Again, molality (m) = $${{Mass\,of\,the\,solute \div \,Molar\,mass} \over {Mass\,of\,the\,solvent}} \times 1000$$

Now, 13% solution of H2SO4 means that 13g of H2SO4 is dissolved in 87g of solvent.

Thus, molality = $${{13/98} \over {87}} \times 1000 = {{13 \times 1000} \over {87 \times 98}} = 1.52$$

$$\therefore$$ Molality of 13% H2SO4 solution = 1.52 (m).

2nd Part : We know, Normality = Molarity $$\times$$ basicity of the acid. Here, basicity of H2SO4 = 2.

$$\therefore$$ N = 1.35 $$\times$$ 2 = 2.70 For dilution, N1V1 = N2V2

$$\therefore$$ 100 $$\times$$ 2.70 N = 1.5 N $$\times$$ V2 or, V2 = $${{100 \times 2.70} \over {1.5}}$$ or, V2 = 180

$$\therefore$$ The sulphuric acid sample should be diluted upto 180 mL to prepare 1.5 N solution.

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