One mole of ideal monoatomic gas $ \left( \gamma = {5 \over 3 } \right) $ is mixed with one mole of diatomic gas $ \left( \gamma = {7 \over 3 } \right) $ What is $\gamma $ for the mixture? $\gamma $ denotes the ratio of specific heat at constant pressure to that at constant volume
$ \gamma _{mix} = { { 1\gamma_1 \over \gamma_1 -1 } + { 2\gamma_2 \over \gamma_2 -1 } \over { 1 \over \gamma_1 - 1 } + {2 \over \gamma_2 -1 } } = { { 1 \times 5/3 \over ( 5/3 - 1 ) } + { 1 \times 7/5 \over 7/5 - 1 } \over {1 \over (5/3 -1)} + {1 \over ( 7/5 -1 ) } }= { 3 \over 2} = 1.5 $