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  4. A sample of 20.0 g of a compound (molecular weight 120) which is a non-electrolyte is dissolved in 10.0 grams of ethanol (C2H5OH) If the vapour pressure of pure ethanol at the temperature used is 0.250 atm, what is the vapour pressure of ethanol above the solution?

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Q. A sample of 20.0 g of a compound (molecular weight 120) which is a non-electrolyte is dissolved in 10.0 grams of ethanol $(C_{2}H_{5}OH)$ If the vapour pressure of pure ethanol at the temperature used is 0.250 atm, what is the vapour pressure of ethanol above the solution?

Solutions

Solution:

$\frac{P^{o}-P_{s}}{P^{o}}=\frac{n}{n+N}$
$\frac{0.25-P_{s}}{0.25}=\frac{\frac{20}{120}}{\frac{20}{120}+\frac{10}{46}}=\frac{0.167}{0.167+0.217}$
$\therefore 0.25-P_{s}=\frac{0.25\times0.167}{0.167+0.217}$
$\therefore P_{s}=0.141$