Question

A solution of an unknown nonvolatile nonelectrolyte was prepared by dissolving 0.250 g of the substance in 40.0 g of CCl4. The boiling point of the resultant solution was 0.357oC higher than that of the pure solvent. Calculate the molar mass of the solute.

Answer 1

Raoult's law states:

$\Delta T = EC_m$, where delta T -- difference between boiling points, E -- ebullioscopic constant (5.0 K*kg/mol for CCl₄), C_m -- molality of solution.

$C_m = m_x/Mm_s$ , where m_x -- mass of the unknown compound, m_s -- mass of the solvent.

Combine the two formulas:

$\Delta T= Em_x/Mm_s$

Find the molar mass:

$M = Em_x/m_s\Delta T$

$M = 5*0.25/(0.04*0.357) = 87.5 (g/mol)$

Answer: 87.5 g/mol is the molar mass of the solute.