Ionisation constant of $ CH_3COOH is 1.7 \times10 ^{-5} and concentration of H ^+ ions is 3.4 \times 10 ^ {-4} The initial concentration of CH_3COOH$ molecules is
$ CH_3 COOH \rightleftharpoons CH_3 COO ^ - + H^+ $ $ at eq (a - 3.4 \times 10 ^ {-4} ) 3.4 \times 10 ^ {-4} 3.4 \times 10 ^ {-4} $ $ { (3.4 \times 10 ^ {-4} ) (3.4 \times 10 ^ {-4} ) \over ( a - 3.4 \times 10 ^ {-4} ) } = 1.7 \times 10 ^ {-5} (Given) $ $ \therefore a = 6.8 \times 10 ^ {-3} $