Phys. Chem. (Chemical Equilibria)

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Homework Statement

From Atkins Phys. Chem 9th Ed, Ex6.5a:

Dinitrogen Tetroxide is 18.46% Dissociated at 25 deg Celcius and 1 bar in the equilibrium N2O4(g) <-> 2NO2(g). Calculate K (equilibrium concentration) at a)25 deg. C. b) 100 deg. C. given that ΔH(standard of reaction) = +56.2 kJmol^-1 over the temperature range

Note: The answer appears in the back of the textbook,
A) 0.141
B) 13.5

Homework Equations

Temperature dependance of Equilibrium constant derived from Van't Hoff:

ΔG=-RT ln(K)

The Attempt at a Solution

Solution was attempted using two separate methods:

Assuming 1 bar of N2O4 present:

K = activity of (NO2)^2 / activity of (N2O4)^2
Using the percentage of dissociation:
K=(0.1846*2)^2/(1-0.1846) = 0.167 (Wrong answer)

Another Attempt:
Using ΔG=-RT*ln(k), ΔG standard for NO2 = 51.31kJmol^-2, for N2O4 = 97.89 kJmol^-1
ΔG for reaction = 2*ΔG(NO2)-ΔG(N2O4) = 4730 J
K=e^(-ΔG/RT) using 8.314 JKmol^-1 for R and 298K for T
K= 0.148 (2nd wrong answer)

I am unable to find the correct answer of .141 using either methods and am unable to proceed to part B)

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