1 . 80   g of solute A was dissolved in 62 . 5   cm 3 of ethanol and freezing point of the…

Given

Mass of ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}=62.5\times 0.8=50 \mathrm{g}$ 

Mass of solute = 1.80 g

Depression in freezing point $∆{\mathrm{T}}_{\mathrm{f}} = 156.0-155.1 = 0.9 \mathrm{K}$

We know that the depression in freezing point

${\mathrm{\Delta T}}_{\mathrm{f}}={\mathrm{K}}_{\mathrm{f}}\times \mathrm{m}$

$0.9=2\times \frac{1.8\times 1000}{{\mathrm{M}}_{\mathrm{w}}\times 50}$

${\mathrm{M}}_{\mathrm{w}}=\frac{2\times 1.8\times 1000}{0.9\times 50}=80$