Question

When 9.72 g of an unknown non-electrolyte is dissolved in 50.0 g of cyclohexane, the boiling point increased to 84.93 degrees C from 80.7 degrees C. If the Kbp of the solvent is 2.79 K/m, calculate the molar mass of the unknown solute.

Answer 1

Answer : The molar mass of unknown compound is 128.22 g/mole

Explanation :

Mass of unknown compound = 9.72 g

Mass of solvent = 50.0 g

Formula used :

`\Delta T_b=i* K_b* m\\\\T_2-T_1=i* K_b*\frac{\text{Mass of unknown compound}}{\text{Molar mass of unknown compound}* \text{Mass of solvent in Kg}}`

where,

`\Delta T_b` = elevation in boiling point

`T_1` = temperature of solvent =
`80.7^oC=273+80.7=353.7K`

`T_2` = temperature of solution =
`84.93^oC=273+84.93=357.93K`

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

`K_f` = boiling point constant for solvent = 2.79 K/m

m = molality

Now put all the given values in this formula, we get:

`(357.93-353.7)K=1* (2.79K/m)* \frac{9.72g* 1000}{\text{Molar mass of unknown compound}* 50.0g}`

`\text{Molar mass of unknown compound}=128.22g/mole`

Therefore, the molar mass of unknown compound is 128.22 g/mole