Dry air is passed through the solution containing the $10 \mathrm{g}$ of solute and $90 \mathrm{g}$ of water, and then it is passed through pure water. The depression in the weight of solution is $2.5 \mathrm{g}$, and that of pure solvent is $0.05 \mathrm{g}$. Calculate the molecular weight of solute.