prepare 0.2M phospate buffer of pH 5.7
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.