Equilibrium

Equilibrium

Question

The pH of a 0.1 molar solution of the acid HQ is 3. The value of the ionisation constant, Ka of this acid is

  • 3 x 10–1

  • 1 x 10–3

  • 1 x 10—5

  • 1 x 10–7

Answer

C.

1 x 10—5

HQ = H+ + Q-
left square bracket straight H to the power of plus right square bracket space equals space square root of straight K subscript straight a straight C end root space by space ostwald apostrophe straight s space dilution space law
left square bracket straight H to the power of plus right square bracket space equals space 10 to the power of negative pH end exponent space equals space 10 to the power of negative 3 end exponent space straight M
straight C space equals space 0.1 space straight M
Thus comma space 10 to the power of negative 3 end exponent space equals space square root of straight K subscript straight a space straight x 0.1 end root
10 to the power of negative 6 end exponent space equals space straight K subscript straight a space straight x space 0.1
straight K subscript straight a space equals space 10 to the power of negative 5 end exponent

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