How much do you want to prepare. I decide that first the I work these. You can decide later if you work it by molarity first. My way here. Let's say you want 1 L, then millimols = mL x M = 1000 mL x 0.2M = 20 mmols.
pH = pK2 + log (base)/(acid)
5.7 = 7.2 + log b/a
b/a = ? = equation 1
b+a = 20 is equation 2.
Solve those two equations simultaneously for acid (H2PO4^-) and base(HPO4^2-).
I would convert mmols to mols, then mols NaH2PO4= grams/molar mass.
Solve for grams for acid.
mols Na2HPO4 = grams/molar mass.
Solve for grams for base.
Take these grams, convert to mols, plug into the HH equation and see if you get 5.7 for pH.
prepare 0.2M phospate buffer of pH 5.7
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