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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.

Solution

Given that

Mass of liquid A , W_A = 100g

Molar mass, M_A = 140 g mol – 1

Mass of liquid B, W_B = 1000 g

Molar mass, M_B = 180 g mol – 1

Use the formula

Number of moles = \frac{mass}{molar mass}

Number of moles of liquid A, M_A = 100/140 = 0.714 mol

Number of moles of liquid B, M_B =1000/ 180 = 5.556 mol

Use formula

Mole fraction of A = \frac{number of moles}{total number of moles}

Molar fraction of A,X_A = 0.714 /(0.714 + 5.556) = 0.114

Similarly

Molar fraction of B, X_B = 1- X_A

= 1 − 0.114 = 0.886

Vapour pressure of pure liquid B, P^o_B = 500 torr

Use formula of Henry`s law

P_B = P^o_B× X_B

Plug the values we get

P_B= 500 × 0.886 = 443 torr

Given that total vapour pressure of the solution, p_total = 475 torr

Use the formula

P_total = p_A + p_B

p_A = p_total − p_B

Plug the values we get

P_A = 475 − 443

P_A = 32 torr

Use formula of Henry`s law again we get

P_A = P^o_A× X_A

Plug the values we get

32 = P^o_A × 0.114

P^o_A = 32/0.114 = 280.7 torr

So that the vapour pressure of pure liquid A = 280.7 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]

472 Views

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) .

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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.

153 Views

Short Answer Type

160. Derive the relationship between relative lowering of vapour pressure and mole fraction of a volatiles liquid.

2177 Views