\(1\) mole of \(CO\) and \(1\) mole of \(O _2\) are mixed.

Net internal energy \(=\frac{f_1}{2} R T_{ CO }+\frac{f_2}{2} R T_{ O _2}\)

\(=\frac{5}{2} R 300+\frac{5}{2} R 350\)

\(=\frac{5}{2} R(650)\)

\(=5 R(325)\)

\(=1625 R\)

\(1625=\frac{5}{2} R T_{\text {final }} \times n_{\text {final }}\)

\(\frac{1625 \times 2}{5}=T_{\text {final }} \times n_{\text {final }}\)

\(325 \times 2=T_{\text {final }} \times 2\)

\(T_{\text {final }}=325 K\)

\(T_{\text {final }}=37^{\circ} C\)