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Question:

Consider the van der Waals constants, a and b, for the following gases: Gas Ar Ne Kr Xe
a/(atm dm⁶mol⁻²) 1.3 0.2 5.1 4.1
b/(10⁻²dm³mol⁻¹) 3.2 1.7 1.0 5.0
Which gas is expected to have the highest critical temperature?

Ar

Ne

Kr

Xe

Solution:

Correct option is A. Kr
Tc = 8a/27Rb
Greater value of a/b, higher is Tc
Gas a/b
Ar = 1.3/3.2 = 0.406
Ne = 0.2/1.7 = 0.118
Kr = 5.1/1 = 5.1
Xe = 4.1/5 = 0.82
∴ Tc has order: Kr > Xe > Ar > Ne