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Electrochemistry

Question
CBSEENCH12010841

The first and second dissociation constants of an acid H2A are 1.0 × 10–5 & 5.0 × 10–10 respectively. The overall dissociation constant of the acid will be

  • 5.0 × 10–5

  • 5.0 × 1015

  • 5.0 × 10–15

  • 0.2 × 105

Solution

C.

5.0 × 10–15

straight H subscript 2 straight A space rightwards harpoon over leftwards harpoon with space on top space HA to the power of minus space plus space straight H to the power of plus
straight K subscript 1 space equals space fraction numerator left square bracket HA to the power of minus right square bracket left square bracket straight H to the power of plus right square bracket over denominator left square bracket straight H subscript 2 straight A right square bracket end fraction space..... space left parenthesis 1 right parenthesis
HA to the power of minus space rightwards harpoon over leftwards harpoon with space on top space straight H to the power of plus space plus space straight A to the power of 2 minus end exponent
straight K subscript 2 space equals space fraction numerator left square bracket straight H to the power of plus right square bracket left square bracket straight A to the power of 2 minus end exponent right square bracket over denominator left square bracket HA to the power of minus right square bracket end fraction space.... space left parenthesis ii right parenthesis
For space space the space reaction
straight K space equals fraction numerator left square bracket straight H to the power of plus right square bracket left square bracket straight A to the power of 2 minus end exponent right square bracket over denominator left square bracket straight H subscript 2 straight A right square bracket end fraction space equals space straight K subscript 1 space straight x space straight K subscript 2
space equals space 1 space straight x space 10 to the power of negative 5 end exponent space straight x space 5 space straight x space 10 to the power of negative 10 end exponent
5 space straight x space 10 to the power of negative 15 end exponent