$C _{ v } \text { for hydrogen }=\frac{5}{2} R$

$C _{ v } \text { for helium }=\frac{3 R }{2}$

$C _{ v } \text { for water vapour }=\frac{6 R }{2}=3 R$

$\therefore \quad\left( C _{ v }\right)_{\text {mix }}$

$\quad=\frac{4 \times \frac{5}{2} R +2 \times \frac{3}{2} R +1 \times 3 R }{4+2+1}=\frac{16}{7} R$

$\therefore C _{ p }= C _{ v }+ R$

$C _{ p }=\frac{16}{7} R + R \text { or } \quad C _{ p }=\frac{23}{7} R$