Question

(16) The vapor pressure of benzene is 53.3 kPa at 60.6 °C, but it fall to 51.5 kPa when 19 g of
a nonvolatile organic compound is dissolved in 500 g benzene. The molar mass of the
nonvolatile compound is
​

Answer 1

The molar mass of the nonvolatile compound is 82.0 g/mol

Explanation:

As the relative lowering of vapor pressure is directly proportional to the amount of dissolved solute.

The formula for relative lowering of vapor pressure will be,

$\frac{p^o-p_s}{p^o}=i\times x_2$

where,

$\frac{p^o-p_s}{p^o}$= relative lowering in vapor pressure

i = Van'T Hoff factor = 1 (for non electrolytes)

$x_2$ = mole fraction of solute  =$\frac{\text {moles of solute}}{\text {total moles}}$

Given : 19 g of nonvolatile organic compound is dissolved in 500 g benzene.

moles of solute = $\frac{\text{Given mass}}{\text {Molar mass}}=\frac{19g}{Mg/mol}$

moles of solvent (benzene) = $\frac{\text{Given mass}}{\text {Molar mass}}=\frac{500g}{78g/mol}=6.4moles$

Total moles = moles of solute  + moles of solvent = $\frac{19g}{Mg/mol}+6.4$

$x_2$ = mole fraction of solute  =$\frac{\frac{19g}{Mg/mol}}{\frac{19g}{Mg/mol}+6.4}$

$\frac{53.3-51.5}{51.5}=1\times \frac{\frac{19g}{Mg/mol}}{\frac{19g}{Mg/mol}+6.4}$

$M=82.0g/mol$

Thus the molar mass of the nonvolatile compound is 82.0 g/mol

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Answer 2

$\mathbb{ \ \ A\ N\ S\ W\ E\ R\: \: }$

The molar mass of the nonvolatile compound is 82.0 g/mol