Liquid Solution

A solution containing 7.5 g of urea (molar mass $=60 \mathrm{~g} \mathrm{~mol}^{-1}$ ) in 1 kg of water freezes at the same temperature as another solution containing 15 g of solute $X$, in the same amount of water. The molar mass of $X\left(\mathrm{~g} \mathrm{~mol}^{-1}\right)$ is