$ P^H of solution obtained by mixing 50ml 0.4 N HCl \& 50ml 0.2 N NaOH $ है
$ 50 ml of 0.4 NHCl = { 0.4 \over 1000 } \times 50 = 0.02 g eq $ $ 50 ml of 0.2 N NaOH = { 0.2 \over 1000 } \times 50 = 0.01 g eq $ NaOH के 0.01 g तुल्यांक, HCl के 0.01 g तुल्यांक को उदासीन करेंगे $ \therefore HCl left unneutralised = 0.01 g eq vol of Sol. =50+50 =100ml$ $ \therefore [HCl] = { 0.01 \over 100} \times 1000 = 0.1 N $ $ or [ H^+] = 0.1 M $ $ \therefore P ^ H = log (0.1) = 1.0 $