Which of the following statements about the composition of the vapour over an ideal 1 : 1 molar mixture of benzene and toluene is correct? Assume that the temperature is constant at 25°C. (Given, vapour pressure data at 25°C, benzene = 12.8 kPa, toluene

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Solution

Q.
Which of the following statements about the composition of the vapour over an ideal 1 : 1 molar mixture of benzene and toluene is correct

Assume that the temperature is constant at 25°C. (Given, vapour pressure data at 25°C, benzene = 12.8 kPa, toluene = 3.85 kPa) [2016]

1 The vapour will contain equal amounts of benzene and toluene.

2 Not enough information is given to make a prediction.

3 The vapour will contain a higher percentage of benzene.

4 The vapour will contain a higher percentage of toluene.

Ans.
(3)

$p_{Benzene} = x_{Benzene} p_{Benzene}^{\circ}$

$p_{Toluene} = x_{Toluene} p_{Toluene}^{\circ}$

For an ideal 1 : 1 molar mixture of benzene and toluene,

$x_{Benzene} = \frac{1}{2}$ and $x_{Toluene} = \frac{1}{2}$

$p_{Benzene} = \frac{1}{2} p_{Benzene}^{\circ} = \frac{1}{2} \times 12.8$ $kPa = 6.4$ $kPa$

$p_{Toluene} = \frac{1}{2} p_{Toluene}^{\circ} = \frac{1}{2} \times 3.85$ $kPa = 1.925$ $kPa$

Thus, the vapour will contain a high percentage of benzene as the partial vapour pressure of benzene is higher as compared to that of toluene.

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