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  4. The vapour pressure of pure benzene at 25° C is 640 mm Hg and that of the solute A in benzene is 630 mm of Hg. The molality of solution is

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Q. The vapour pressure of pure benzene at $25^\circ C$ is 640 mm Hg and that of the solute A in benzene is 630 mm of Hg. The molality of solution is

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Solution:

$\text{P} \, \text{=} \, \text{P}^{\text{o}} \text{X}_{\text{A}}$

$630=640X_{A}$

$X_{A}=\frac{630}{640}=0.984$

$X_{B}=0.016$

$m=\frac{X_{B} \times 1000}{X_{A} M_{A}}=\frac{0.016 \times 1000}{0.984 \times 78}$

$=0.2 \,m$