# Entropy change in mixing a gas at constant temperature and pressure

The expression given on Wikipedia is (https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwj-_pDA2IfSAhUFto8KHa2kBAwQygQIIDAA&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FEntropy_of_mixing%23Mixing_of_ideal_materials_at_constant_temperature_and_pressure&usg=AFQjCNFVf8mAoquhiL05HeVXC1H3cw54xA) But If I evaluate it as following I get a different answer . $$\Delta S=\int q_{rev}/T=\int pdV/T$$ as p and T are constants $$p(V_1+V_2)/T$$ What is wrong with my method ?