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Question:

Two vessels separately contain two ideal gases A and B at the same temperature, the pressure of A being twice that of B. Under such conditions, the density of A is found to be 1.5 times the density of B. The ratio of molecular weight of A and B is:

12

23

34

2

Solution:

For ideal gas, PV=RT and M=ρV thus, PM/ρ=RT
There, PAMA/ρA=RT (1) and PBMB/ρB=RT (2)
(1)/(2) ⇒ MA/MB = ρA/ρB × PB/PA = (3/2) × (1/2) = 3/4