Question

If a weak acid is 25 deprotonated at ph 4 what would the pka be

Answer 1

4.48

pH=pKa+log([A-/HA])

25% deprotonated tells us that A- is .25 and that the rest (75% is protonated) thats .75.

4 = pKa + log \frac{.25}{.75}

4 - log \frac{.25}{.75}  = pKa

4.48=pKa