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The Henderson-Hasselbalch relationship describing each ionizable group is: $$\mathrm{pH} = \mathrm{p}K_\mathrm{a} + \log \frac{\ce{[A-]}}{\ce{[AH]}}$$ We can solve for the ratio: $$10^{(\mathrm{pH} - \mathrm{p}K_\mathrm{a})}= \frac{\ce{[A-]}}{\ce{[AH]}}$$ However, we really want the fraction of protonated among the total (not the ratio of deprotonated ...
Below a more general approach. Suppose that we have two weak acids $\ce{HA}$ and $\ce{HB}$. The initial concentrations are $C^0_\ce{HA}$ and $C^0_\ce{HB}$, and their constants are ${K_\ce{a}}_\ce{(HA)}$ and ${K_\ce{a}}_\ce{(HB)}$. Suppose yet that volumes, $V_\ce{HA}$ and $V_\ce{HB}$, are additives. So we have: Reactions \ce{HA + H2O <=> H3O+ + A-}...