The mass of a non-volatile solute(in gm) of molar mass 40gmol- that should be dissolved in 114 g of octane to lower its vapour pressure by 20% is: (assume solution is dilute)

# The mass of a non-volatile solute(in gm) of molar mass $40{\mathrm{gmol}}^{-}$ that should be dissolved in 114 g of octane to lower its vapour pressure by is: (assume solution is dilute)

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### Solution:

Assume that the vapor pressure of pure octane $\left({\mathrm{C}}_{8}{\mathrm{H}}_{18}\right)$ is  ${p}_{A}^{°}$ of pure solvent and the vapor pressure of octane after dissolving non-volatile solution is  $\frac{80}{100}{p}_{A}$.

Applying formula:

When solution is dilute

$\frac{{\mathrm{p}}_{\mathrm{A}}^{°}-{\mathrm{p}}_{\mathrm{A}}}{{\mathrm{p}}_{\mathrm{A}}^{°}}=\frac{{n}_{solute}}{{n}_{solvent}}$

Substitute the values:

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