MCQ Solution: Mole Concept

When 2 g of a gas A is introduced into an evacuated flask kept at 25°C, the pressure is found to be 1 atm. If 3g of another gas B is then added to the same flask, the pressure becomes 1.5 atm. Assuming ideal gas behavior, find the ratio of molecular masses MA : MB.

Select one:

1) 1∶2
2) 3∶1 
3) 2∶1
4) 1∶3

Number of mol of A = 2/MA and those of B = 3/MB

Pressure due to 2 / MA mol of A = 1.0 atm

Pressure due to 3 / Mmol of = 1.5 - 1.0 = 0.5 atm

Since, pressures are proportional to molar concentrations of the gaseous substances,


2 / MA / 3/M= 1.00.5

⇒M/ M= 1 / 3

The correct answer is: 1∶3

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