Heat supplied \(=\Delta U=U_{\text {final }}-U_{\text {initial }}\)

Total internal energy initially \(=\frac{5}{2} N R T\)  [Only diatomic gas is present]

Total internal energy when

' \(n\) ' moles get dissociated \(=\frac{5}{2}(N-n) R T+\frac{3}{2}(2 n) R T\) [diatomic and monoatomic both are present]

\(\Delta U=\left\{\frac{5}{2}(N-n) R T+\frac{3}{2}(2 n) R T\right\}-\frac{5}{2} N R T\)

Solving this we get

\(\Delta U=\frac{1}{2} n R T\)

\(\therefore\) Heat supplied is \(\frac{1}{2} n R T\).