Question

A solution containing 7 g of a non-volatile solute in 250 g of water boils at 373.26 K. Find the molecular mass of the solute.

Answer 1

Answer:

Hence, the required mass of the solute is 8 g. Therefore, 23 g mol^{-1} is the molar mass of the solute. Hence, 3.53 KPa is the vapour pressure of water at 298 K

Answer 2

The molecular mass of the solute is 56 g/mol

Explanation:

Elevation in boiling point is given by:

$\Delta T_b=i\times K_b\times m$

$\Delta T_b=T_b-T_b^0=(373.26-373)K=0.26K$ = elevation in boiling point

i = vant hoff factor = 1 (for non electrolyte)

$K_b$ = boiling point constant for water = $0.52K/kgmol$

m= molality

$\Delta T_b=i\times K_b\times \frac{\text{mass of solute}}{\text{molar mass of solute}\times \text{weight of solvent in kg}}$

Weight of solvent (water)= 250 g = 0.25 kg

Molar mass of unknown non volatile solute = M g/mol

Mass of unknown non volatile solute added = 7 g

$0.26=1\times 0.52\times \frac{7g}{M g/mol\times 0.25kg}$

$M=56g/mol$

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