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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ig110

A gas mixture of $He$ (4) and $O X_{2}$ (32) has an average molar mass of $11 g mo l^{- 1}$ . Which are correct?

AThe mole fraction of He is $0.75$
BThe mass fraction of He is $3/11$
CAt STP the density of the mixture is about $0.49 g L^{- 1}$ (molar volume $22.4 L$ )
DThe partial pressure of $O X_{2}$ is three times that of He

Answer: A, B, C

4 x + 32 (1 - x) = 11

,

28 x = 21

,

x_{He} = 0.75

(A). Per mole of mixture: He

3 g

,

O X_{2}

8 g

, so He mass fraction

= 3/11

(B).

d = 11/22.4 = 0.491 g L^{- 1}

(C). Partial pressure follows mole fraction, so

p_{He}

is three times

p_{O X_{2}}

, the reverse of (D).

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