Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 13

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Q: 30. Calculate the amount of benzoic acid required for preparing of solution in methanol.

Answer:

solution of benzoic acid in methanol means,

of solution contains mol of benzoic acid

Therefore, mL of solution contains of benzoic acid

of benzoic acid

Molar mass of benzoic acid

Hence, required benzoic acid

Q: 31. The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.

Q 31 Image of Acetic Acid

Q 31 Image of Acetic Acid

Q 31 Image of Acetic Acid

Q 31 1 Image of Trichloroacetic Acid

Q 31 1 Image of Trichloroacetic Acid

Q 31 1 Image of Trichloroacetic Acid

Q 31 2 Image of Trifluoroacetic Acid

Q 31 2 Image of Trifluoroacetic Acid

Q 31 2 Image of Trifluoroacetic Acid

Among H, , and F, H is least electronegative while F is most electronegative. Then, F can withdraw electrons towards itself more than and H. Thus, trifluoroacetic acid can easily lose ions i.e., trifluoroacetic acid ionizes to the largest extent. Now, the more ions produced, the greater is the depression of the freezing point. Hence, the depression in the freezing point increases in the order:

Acetic acid < trichloroacetic < trifluoroacetic acid

Q: 32. Calculate the depression in the freezing point of water when 10 g of is added to of water. , .

Answer:

Molar mass of

∴ No. of moles present in 10 g of

It is given that 10 g of is added to of water.

∴ Molality of the solution,

Let α be the degree of dissociation of

undergoes dissociation according to the following equation:

Since αis very small with respect to

Now,

Again,

Total moles of equilibrium

Hence, the depression in the freezing point of water is given as: