Initial energy of gas = Final energy

Let K.E. of each molecule initially be \(E_0\)

\(\therefore\) Total kinetic energy \(=E_0 \times n\)

Let final kinetic energy of each molecule be \(E_f\)

\(E_0 \times n=E_f \times 2 n\)

\(E_f=\frac{E_0}{2}\)

Since temperature is the average kinetic energy of molecules.

\(T_0=K E_0 \quad T_f=\frac{K E_0}{2}\)

\(\therefore\) Temperature becomes \(T_f=\frac{T_0}{2}\)