Question

Two aqueous solution containing, respectively, 7 g urea (molar mass = 60 g) and 42 g of substance $X$ in 100 g of water freeze at the-same temperature. Calculate the molecular weight of $X$.

Answer 1

$△T_f=molality\times K_f$

Since $△T_f$ is same for both solutions,

i.e. $△T_{f_1}-△T_{f_2}$(given)

$molalit{y}_{\left(urea\right)}\times K_f=molalit{y}_{\left(x\right)}\times K_f$

$\frac{\frac{7}{60}\times 1000}{100}=\frac{\frac{42}{M_{w\left(x\right)}}\times 1000}{100}$

$M_w=\frac{42\times 60}{7}=360gm$