Equilibrium

Equilibrium

Question

The pKa of a weak acid (HA) is 4.5. The pOH of an aqueous buffered solution of HA in which 50% of the acid is ionized is

  • 4.5

  • 2.5 

  • 9.5

  • 7.0

Answer

C.

9.5

For buffer solution
PH space equals space pKa space plus space log space fraction numerator left square bracket salt right square bracket over denominator left square bracket Acid right square bracket end fraction
equals space 4.5 space plus space log space fraction numerator left square bracket salt right square bracket over denominator left square bracket Acid right square bracket end fraction
as HA is 50% ionized so [Salt] = [Acid]
pH = 4.5 pH + pOH = 14
⇒ pOH = 14 – 4.5 = 9.5

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