Question

The vapor pressure of benzene is $53.3k{P}_a$ at $603{}^{o}C$ , but it falls to $\text{51}\text{.5}{\text{kP}}_{\text{a}}$ when $\text{19}\text{g}$ of a non-volatile organic compound is dissolved in $\text{500}\text{g}$ benzene. The molar mass of the non-volatile compound is

• A. 82
• B. 85
• C. 88
• D. 92

Answer 1

Answer: (B)

${\text{P}}^{\text{0}}\text{=}\text{53}\text{.3}\text{kPa}$

${\text{P}}_{\text{s}}=51.5{\text{kP}}_{\text{a}}$

$\frac{P^0-P_s}{P_s}=\frac{n}{N}=\frac{19\times 78}{M\times 500}$

$\frac{53.3-51.5}{51.5}=\frac{n}{N}=\frac{19\times 78}{M\times 500}$

$M=\frac{19\times 78\times 51.5}{(53.3-51.5)\times 500}$

$M=85g$