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151. Benzene and toluene form iedal solution over the entire range of composition. The vapour pressure of pure benzene and naphthalene at 300 K are 50.71 mm Hg and 32.06 mm Hg respectively. Calculate the mole-fraction of benzene in vapour phase if 80 g of benzene is mixed with 100 g of toluene.

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152. 100 g of liquid A (molar mass 140 g mol^–1) was dissolved in 1000 g of liquid B (molar mass 180 g mol^–1). The vapour pressure of pure liquid B and found to be 500 torr. Calculate the vapour pressure of pure liquid A and its vapour pressure in the solution if the total vapour pressure of the solution is 475 torr.

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153.

Heptane and Octane form an ideal solution at 373 K. The vapour pressures of the pure liquids at this temperature are 105.2 K Pa and 46.8 K Pa respectively. If the solution contains 25 g of heptane and 28.5 g of octane, calculate
(i) Vapour pressure exerted by heptane.
(ii) Vapour pressure exerted by solution.
(iii) Mole fraction of octane in the vapour phase.

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Short Answer Type

154. Ethylene glycol (molar mass = 62 g mol^–1) is a common automobile auto freeze. Calculate the freezing point of a solution containing 12.4 g of this substance in 100 g of water. Would it be advisable to keep the substance in the car radiator during summer?

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155. Calculate the temperature at which a solution containing 54 g of glucose, C₃H₁₂O₆, in 250 g of water will freeze. [K_ffor water = 1.86 K kg mol^–1]

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Long Answer Type

156. A solution containing 8 g of a substance in 100 g of diethyl ether boils at 36.86⁰C, whereas pure ether boils at 35.60⁰C. Determine the molecular mass of the solute. (For ether K_b= 2.02 K kg mol^–1) .

Answer;

We have given that

∆ T_{b} = (36.86 - 35.60) = 1.26 K M_{B} = ?, W_{B} = 8 g, W_{A} = 100 g k_{b} = 2.202 K kg {mol}^{- 1}

k_{b} = \frac{∆ T_{b} \times M_{B} W_{A}}{W_{b} \times 1000}

Therefore

2.02 = \frac{1.26 \times M_{B} \times 100}{8 \times 1000}

M_{B} = \frac{2.02 \times 8 \times 1000}{1.26 \times 100} = 128.25 .

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157. How is osmotic pressure of a solution determined ? If the membrane used was slightly leaky, how will it influence the measured value of osmotic pressure?
Osmotic pressure of a solution containing 7 g of a protein per 103 ml of solution is 25 mm Hg at 310 K. Calculate the molecular mass or the protein. (R = 0.0821 L atm K^–1 mol).

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158. Define vapour pressure of a liquid. What happens to the vapour pressure when (a) a volatile solute dissolves in the liquid and (b) the dissolved solute is non-volatile?

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159. Discuss the various types of plots between the partial vapour pressure and the mole fractions of two components of the completely miscible liquids in a solution.

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