CBSE Class 12 · 2024 · SolutionsLong Answer

(b)(i) The boiling point of water at 750 mm Hg pressure is 99.68°C. How much sucrose (molar mass = 342 g mol

^{- 1}

) is to be added to 500 g of water so that it boils at 100°C? (K

_{b}

for water = 0.52 K kg mol

^{- 1}

)
(ii) State Henry's law and write any one application of it.

Answer: (i) ≈ 105.2 g (ii) p = K_H x; e.g. soft drinks are bottled under high CO₂ pressure to increase its solubility.

Solution

(i) Required elevation:

Δ T_{b} = 100 - 99.68 = 0.32 K

Required molality:

m = \frac{0.32}{0.52} = 0.6154 mol kg^{- 1}

Moles needed in 500 g water =

0.6154 \times 0.500 = 0.3077

mol.
Mass of sucrose:

w = 0.3077 \times 342

w = 105.2 g

(ii) Henry's law: at constant temperature, the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid,

p = K_{H} x

.
Application: soft drinks and soda water are bottled under high CO

_{2}

pressure to keep more of the gas dissolved. (Equally acceptable: He–O

_{2}

breathing mixtures for deep-sea divers.)

CBSE Class 12 Board · 2024 · Subjective