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ICSE Class 9 Chemistry Question 16 of 29

Study of Gas Laws — Question 25

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Question

Question 25

50 cm3 of hydrogen is collected over water at 17°C and 750 mm Hg pressure. Calculate the volume of dry gas at S.T.P. The water vapour pressure at 17°C is 14 mm Hg.

Answer

Initial conditions :

P1 = 750 - water vapour pressure = 750 - 14 = 736 mm Hg

V1 = Initial volume of the gas = 50 cm3

T1 = Initial temperature of the gas = 17°C = 17 + 273 = 290 K

Final conditions :

P2 (Final pressure) = 760 mm of Hg

T2 (Final temperature) = 273 K

V2 (Final volume) = ?

By Gas Law:

P1×V1T1=P2×V2T2\dfrac{\text{P}_1\times\text{V}_1}{\text{T}_1} = \dfrac{\text{P}_2\times\text{V}_2}{\text{T}_2}

Substituting the values :

736×50290=760×V2273V2=736×50×273290×760V2=45.58 cm3\dfrac{736 \times 50}{290} = \dfrac{760 \times \text{V}_2}{273} \\[1em] \text{V}_2 = \dfrac{736 \times 50 \times 273}{290 \times 760} \\[1em] \text{V}_2 = 45.58 \text{ cm}^3 \\[1em]

∴ Volume of dry gas = 45.6 cm3