MadSci Network: Biochemistry |

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Hi Patty, If I understand your question correctly, you are having trouble using all the information you have been given with the Henderson-Hasselbach equation. So, as you mentioned, you know the pH and pKa values for the equation, but cannot convert the [A-]/[HA] ratio into actual concentrations. That's where a second equation comes into play. So, in your example, you know that [A-] + [HA] = 0.1M, or 0.1 - [A-] = [HA]. If you insert this information into the H.H. equation, you can solve for one of the concentrations. So, in the case of Arginine, the first pKa is 1.82 (for COOH). The H.H. equation then becomes 4 = 1.82 + log ([A-]/([0.1 - [A-])), or 2.18 = log ([A-]/([0.1 - [A-])). This rearranges to 10^(2.18) = ([A-])/(0.1 - [A-]), which you can rearrange further to get [A-]. Once you know [A-], you substitute this value back into [A-] + [HA] = 0.1M to get the value of [HA]. And so on for all your calculations. Hope this helps!

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