Question

Joe is given a 118 g sample of an impure aqueous solution. The solution does not conduct electricity so he assumes the impurity is a covalent nonelectrolyte. From density measurements the sample appears to contain 100.0 g of water. Additionally, he was able to freeze the sample at -1.80 ºC. What is the molar mass of the impurity?

Answer 1

Answer: 186 g/mol

Explanation:

Weight of solvent (water)=100 g = 0.1 kg      (1 kg=1000 g)

Molar mass of solute (impurity) = ?

Mass of solute (impurity) added = mass of solution - mass of solvent (water) = (118- 100) = 18 g

$\Delta T_f=K_f\times \frac{\text{mass of solute}}{\text{molar mass of solute}\times \text{weight of solvent in kg}}$

$\Delta T_f$ = change in freezing point

$K_f$ = freezing point constant for water = $1.86^0C/m$

$\Delta T_f=T_f^0-T_f=(0-(1.80))^0C=1.80^0C$

$1.80=1.86\times frac{18}{M\times 0.1}$

$M=186g/mol$

The molecular mass of the impurity is 186 g/mol.