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07/09/2021 12:02 pm
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When 2 g of a gas A is introduced into an evaluated flask kept at 25°C, the pressure is found to be one atmosphere. If 3 gm of another gas B is then added to the same flask, the total pressure becomes 1.5 atm. Assuming ideal gas behaviour, calculate the ratio of the molecular weights MA : MB.
(a) 1 : 3
(b) 1 : 1
(c) 2 : 1
(d) 3 : 1
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07/09/2021 12:23 pm
Correct answer: (a) 1 : 3
Explanation:
From ideal gas equation, PV = nRT
PV = (\(\frac{m}{M}\))RT or M = m\(\frac{RT}{PV}\)
Let the molecular wt. of A and B be MA and MB respectively.
Then MA = 2 x \(\frac{RT}{1 \times V}\); MB = \(\frac{3RT}{0.5 \times V}\)
∴ \(\frac{M_A}{M_B}\) = \(\frac{2RT}{V}\) x \(\frac{0.5V}{3RT}\)
= \(\frac{2 \times 0.5}{3}\) = \(\frac{1}{3}\)
Therefore, the ratio MA : MB = 1 : 3