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Calculating Acid Dissociation Constant

A diprotic acid, H2A, has the following ionization constants: Ka1 = 1.1E-3 and Ka2 = 2.5E-6. In order to make up a buffer solution of pH 5.8, which combination would you choose, NaHA/H2A or Na2A/NaHA? I know the solution is Na2A/NaHA, but I am not sure why. What is the pKa of the acid component?

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Solution Summary

This solution is provided in 52 words in an attached .doc file. It shows the buffer equation and uses this to find the ratio of acid and base. pKa is calculated using a simple log formula.