IIT-JEE 2011 · Paper 1 · Chemistry Q21

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ChemistryInteger+4Question 21 of 69

To an evacuated vessel with movable piston under external pressure of 1 atm., 0.1 mol of He and 1.0 mol of an unknown compound (vapour pressure 0.68 atm. at 0 °C) are introduced. Considering the ideal gas behaviour, the total volume (in litre) of the gases at 0 °C is close to

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Answer and worked solution

Official answer: 7

Partial pressures. The vapour of the compound is capped at its vapour pressure, 0.68 atm (most of it stays liquid); with the piston at 1 atm, helium supplies 0.32 atm.

Volume. $V=\dfrac{n_{He}RT}{p_{He}}=\dfrac{0.1\times0.0821\times273}{0.32}=7.0$ L.

(Check: the vapour then holds $\frac{0.68\times7}{22.4}\approx0.21$ mol, less than the 1.0 mol present.)

Answer: 7

Topics:Raoult's law and vapour pressure

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IIT-JEE 2011 · Paper 1, Chemistry question 21