Question

A solution prepared by dissolving a $2.50g$ sample of an unknown compound dissolved in $34.0g$ of benzene,
$C_6{H}_6$ boils at ${1.38}^{\circ }C$ higher than pure benzene. Which expression gives the molar mass of the unknown compound
?
Compound $K_b$
$C_6{H}_6{2.53}^{\circ }C.m^{-1}$

• A. $2.53\times \frac{2.50}{1.38}$
• B. $1.38\times \frac{34.0}{2.53}\times 2.50$
• C. $2.50\times {10}^3\times \frac{2.53}{34.0}\times \frac{1}{1.38}$
• D. $2.50\times {10}^3\times \frac{1.38}{34.0}\times 2.53$

Answer 1

Answer: (C)

$2.50\times {10}^3\times \frac{2.53}{34.0}\times \frac{1}{1.38}$

As we know,
$M_2=\frac{1000\times K_b\times w_2}{\Delta T_b{w}_1}$
$M_2=\frac{1000\times 2.53\times 2.5}{1.38\times 34}$