Ratio of $C_p/C_v$ for ${\mathrm{NH}}_3$ gas (assuming ideal behaviour) when vibrational degree of freedom are active:

$$\begin{gathered} \begin{array}{l}{C}_V=\frac{1}{2}\times R(T+R+2V)\\ =\frac{1}{2}\times R(3+3+2\times 6)\\ =\frac{R}{2}\times 18 \\ \Rightarrow 9R\\ C_V=9R\end{array} \end{gathered}$$

Ideal gas behavior

$\begin{array}{l}{C}_p-C_V=R\\ C_p=C_V+R\\ C_p=9R+R\\ C_p=10R\end{array}$

$\frac{CP}{CV}=r=\frac{10R}{9R}=\frac{10}{9}$

$C_v=\frac{3R}{2}+\frac{3R}{2}+6R=9R,C_p=10R$