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Phosphoric acid is a triprotic acid with the following pKa values:
pKa1=2.148 pKa2=7.198 pKa3=12.375

You wish to prepare 1.000 L of a 0.0200 M phosphate buffer at pH 7.11. To do this, you choose to use mix the two salt forms involved in the second ionization, NaH2PO4 and Na2HPO4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture?

  • chemistry -

    You need two equations and solve them simultaneously.
    1000 mL x 0.02M = 20 millimols.
    mols acid + mols base = 20

    pH = pKa + log(base)/(acid)
    7.0 = pK2 + log (base)/(acid) and
    you solve for (base)/acd) = ?
    Then solve the two equations for mols acid and mols base, convert mols each to grams for 1 L. Post your work if you get stuck.

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