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  4. 1.80 g of solute A was dissolved in 62.5 cm 3 of ethanol and freezing point of the solution was found to be 155.1 K. The molar mass of solute A is g mol -1. [Given: Freezing point of ethanol is 156.0 K. Density of ethanol is 0.80 g cm -3. Freezing point depression constant of ethanol is .2.00 K kg mol -1]

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Q. $1.80\, g$ of solute A was dissolved in $62.5\, cm ^3$ of ethanol and freezing point of the solution was found to be $155.1\, K$. The molar mass of solute $A$ is___ $g \, mol ^{-1}$.
[Given: Freezing point of ethanol is $156.0 \, K$.
Density of ethanol is $0.80 \, g \, cm ^{-3}$.
Freezing point depression constant of ethanol is $\left.2.00 \, K\, kg\, mol ^{-1}\right]$

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Solution:

Mass of $C _2 H _5 OH =62.5 \times 0.8=50 g$
$ \Delta T _{ f }= K _{ f } \times m $
$0.9=2 \times \frac{1.8 \times 1000}{ M _{ w } \times 50}$
$ M _{ w }=\frac{2 \times 1.8 \times 1000}{0.9 \times 50}=80$