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Question

The equilibrium constant Kp for the reaction
N2O4(g)2NO2(g) is 4.5
What would be the average molar mass (in g/mol) of an equilibrium mixture of N2O4 and NO2 formed by the dissociation of pure N2O4 at a total pressure of 2 atm?

A
69
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B
57.5
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C
80.5
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D
85.5
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Solution

The correct option is B 57.5
Let initial mole of N2O4(g) is one
N2O4(g)2NO2(g)
initial moles 10
At equilibrium1α2x
Total number of moles at equilibrium =1+α
PN2O4=1α1+α×P
PNO2=2α(1+α)×P
Hence,
KP=P2NO2PN2O4=4α2(1α2)×2
4.5=4α2(1α2)×2
α=0.6
Mole fraction of N2O4:
XN2O4=1α1+α=0.25
XNO2=0.75
Average molar mass of mixture
=0.25×92+0.75×46=57.5

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