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

Step-by-step 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* \frac{\text{mass of solute}}{\text{molar mass of solute}* \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* frac{18}{M* 0.1}`

`M=186g/mol`

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