Answer
B has a molar mass of 60, A has a molar mass of 30
Work Step by Step
We are given two flasks with equal volumes and they are both at the same temperature. From the ideal gas law, we have:
PV = nRT
If V and T are constant, it means only the number of moles is different.
We are told the pressure of A is Xatm and B is 0.5X atm. Since the pressure in B is half of the pressure in A, then B must have half the number of moles of substance.
They both have 1.2g so that means the molar masses must be different. Since B has half the number of moles, the molar mass must be twice as large so:
B has a molar mass of 60, A has a molar mass of 30