Question

In a laboratory experiment, students synthesized a new compound and found that when 7.229 grams of the compound were dissolved in 207.8 grams of chloroform, the vapor pressure of the solution was 170.51 mm Hg. The compound was also found to be nonvolatile and a non-electrolyte. What is the molecular weight of this compound?

Answer 1

The question is incomplete, here is the complete question:

The common laboratory solvent chloroform is often used to purify substances dissolved in it. The vapor pressure of chloroform, CHCl₃, is 173.11 mm Hg at 25°C.

In a laboratory experiment, students synthesized a new compound and found that when 7.229 grams of the compound were dissolved in 207.8 grams of chloroform, the vapor pressure of the solution was 170.51 mm Hg. The compound was also found to be nonvolatile and a non-electrolyte. What is the molecular weight of this compound?

Answer: The molecular mass of the compound is 481.9 g/mol

Explanation:

The equation used to calculate relative lowering of vapor pressure follows:

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

where,

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

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

$\chi_{solute}$ = mole fraction of solute = ?

$p^o$ = vapor pressure of pure chloroform = 173.11 mmHg

$p_s$ = vapor pressure of solution = 170.51 mmHg

Putting values in above equation, we get:

$\frac{173.11-170.51}{173.11}=1\times \chi_A\\\\\chi_A=0.0150$

This means that 0.0150 moles of compound is present in the solution

To calculate the number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

Moles of compound = 0.0150 moles

Given mass of compound = 7.229 g

Putting values in above equation, we get:

$0.0150mol=\frac{7.229g}{\text{Molar mass of compound}}\\\\\text{Molar mass of compound}=\frac{7.229g}{0.0150mol}=481.9g/mol$

Hence, the molecular mass of the compound is 481.9 g/mol