#### 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