The vapour pressure of pure liquid A is $10 torr$ and at the same temperature, when $1 g$ of $\mathit{B}$ (non-volatile solid) is dissolved in $20 g$ of $A$, it's vapour pressure is reduced to $9.0 torr$. If the molecular mass of $A$ is $200 amu$, then the molecular mass of $\mathit{B}$ is :