posted by Laura .
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?
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.