Heptane and octane form ideal solution. At 373 K, the vapour pressures of the two liquids are 105.2 kPa and 46.8 kPa respectively. What will be the vapour pressure, in bar, of a mixture of 25 g heptane and 35 g of octane?

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Solution

(A) Heptane $C_{7} H_{16} m_{A} = 100$
(B) Octane $C_{8} H_{18} m_{B} = 114$
$n_{A} = \frac{w_{A}}{m_{A}} = \frac{25}{100} = 0.25; n_{B} = \frac{35}{114} = 0.3$
$x_{A} = \frac{0.25}{0.25 + 0.30} = 0.45; x_{B} = \frac{0.3}{0.25 + 0.30} = 0.55$
$p = p_{A}^{0} x_{A} + p_{B}^{0} x_{B}$
$= 105.2 \times 0.45 + 46.8 \times 0.55$
$= 47.34 + 25.74 = 73.08 k P a$

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