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Question:

Two moles of helium gas is mixed with three moles of hydrogen molecules (taken to be rigid). What is the molar specific heat of mixture at constant volume?

21.6 J/mol K

19.7 J/mol K

17.4 J/mol K

15.7 J/mol K

Solution:

Correct option is C. 17.4 J/mol K

fmix = (n1f1 + n2f2)/(n1 + n2) = (2 × 3 + 3 × 5)/5 = 21/5
Cv = fR/2 = (21/5) × R/2 = 17.4 J/mol K