Question

4 moles of hydrogen, 2 moles of helium and 1 mole of water vapour form an ideal gas mixture. What is the molar specific heat at constant pressure of mixture?

• A. $\frac{16}{7}R$
• B. $\frac{7R}{16}$
• C. R
• D. $\frac{23}{7}R$

Answer 1

Answer: (D)

$\frac{23}{7}R$

A monoatomic molecule has 3 degrees of freedom, a diatomic molecule has 5 degrees of freedom and a triatomic molecule has 6 degrees of freedom.

Thus the average degree of freedom is $\frac{\left(5\right)\left(4\right)+\left(3\right)\left(2\right)+\left(6\right)\left(1\right)}{4+2+1}=\frac{32}{7}$

$C_v=\frac{f}{2}R=\frac{16}{7}R$

Thus $C_p=C_v+R=\frac{23}{7}R$