P and Q combine to form two compounds PQ2 and PQ3. If 1 g of PQ2 is dissolved in 51 g of benzene, the depression of freezing point becomes 0.8°C. On the other hand, if 1 g of PQ3 is dissolved in 51 g of benzene, the depression of freezing point becomes 0.625°C. The atomic mass of P and Q are (Kf of benzene = 5.1 K·kg/mol):