Question

A solution containing 8 g of a substance in 100 g of diethyl ether
boils at 36.86 °C whereas pure ether boils at 35.60°C. Determine
the molecular mass of the solute. (K, for ether = 2.02 K kg mol-ly

Answer 1

The molecular mass of the solute is 128 g/mol

Explanation:

Elevation in boiling point is given by:

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

$\Delta T_b=T_b-T_b^0=(36.86-35.60)^0C=1.26^0C$ = Elevation in boiling point

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

$K_b$ =boiling point constant = $2.02K/kgmol$

m= molality

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

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

Molar mass of unknown non electrolyte = M g/mol

Mass of unknown non electrolyte added = 8 g

$1.26=1\times 2.02\times \frac{8g}{M g/mol\times 0.1kg}$

$M=128g/mol$

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