Question

The ionisation constant of chloroacetic acid is 1·35 × 10^–3. What will be the pH of 0·1M acid and its 0·1M sodium salt solution?

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Solution

Chloroacetic acid ionises as,

ClCH subscript 2 COOH space space rightwards harpoon over leftwards harpoon space space ClCH subscript 2 COO to the power of minus space plus space straight H to the power of plus therefore space space space space space straight K subscript straight a space equals space fraction numerator open square brackets ClCH subscript 2 COO to the power of minus close square brackets space open square brackets straight H to the power of plus close square brackets over denominator open square brackets ClCH subscript 2 COOH close square brackets end fraction In space this space case comma space open square brackets ClCH subscript 2 COO to the power of minus close square brackets space equals space open square brackets straight H to the power of plus close square brackets therefore space space space straight K subscript straight a space equals space open square brackets straight H to the power of plus close square brackets squared over straight C space therefore space space space space open square brackets straight H to the power of plus close square brackets space equals space square root of straight K subscript straight a space cross times space straight C end root

equals space square root of 1.35 space cross times space 10 to the power of negative 3 end exponent space cross times space 0.1 end root equals space 1.16 space cross times space 10 to the power of negative 2 end exponent

But space space space pH space equals space minus log space open square brackets straight H to the power of plus close square brackets space equals space minus log space left parenthesis 1.16 space cross times space 10 to the power of negative 2 end exponent right parenthesis space space space space space space space space space space space space space equals space 2 minus log space 1.16 space equals space 2 minus 0.06 space equals space 1.94 space space space space space space space space space space space space space

Since sodium salt of chloroacetic acid is a salt of strong base and weak acid, therefore,

pH space equals space minus 1 half open square brackets log space straight K subscript straight w space plus space log space straight K subscript straight a space minus space log space straight C close square brackets space space space space equals negative 1 half open square brackets log space 10 to the power of negative 14 end exponent space plus space log space 1.35 space cross times space 10 to the power of negative 3 end exponent space minus space log space 0.1 close square brackets space space space space space equals negative 1 half open square brackets negative 14 plus left parenthesis negative 3 plus 0.1303 close square brackets space minus left parenthesis negative 1 right parenthesis right square bracket space space space space space equals space 7 plus 1.44 minus 0.5 space space space space space space equals space 7.94

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