25 mL of 0.10 M weak acid HA is mixed with 10 mL of 0.10 M NaOH. The measured pH is 4.52. Given $lo g 1.5 = 0.176$ , find $p K_{a}$ of HA to two decimals.

Numerical answer type, tolerance 0.01.

Answer: 4.6961

HA = 2.5 mmol, NaOH = 1.0 mmol gives HA = 1.5 mmol, A

^{-}

= 1.0 mmol.

p K_{a} = pH - lo g (1.0/1.5) = 4.52 + 0.176 = 4.696

, i.e. 4.70.