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  4. The correct option for the value of vapour pressure of a solution at 45° C with benzene to octane in molar ratio 3: 2 is: [At 45° C vapour pressure of benzene is 280 mm Hg and that of octane is 420 mm Hg. Assume Ideal gas]

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Q. The correct option for the value of vapour pressure of a solution at $45^{\circ} C$ with benzene to octane in molar ratio $3: 2$ is : [At $45^{\circ} C$ vapour pressure of benzene is $280\, mm\, Hg$ and that of octane is $420\, mm \,Hg$. Assume Ideal gas]

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Solution:

Applying Raoult's law,
$P_{\text {total }}=P_{A}^{0} . X_{A}+P_{B}^{0} . X_{B}$
$X_{A}=\frac{3}{5}, X_{B}=\frac{2}{5}$
$P_{\text {total }}=280 \times\left(\frac{3}{5}\right)+420 \times\left(\frac{2}{5}\right)$
$=336 \,mm$ of $ Hg$