Phosphoric acid is a triprotic acid with the following pKa values: pK 2.148 pK7.
ID: 997268 • Letter: P
Question
Phosphoric acid is a triprotic acid with the following pKa values: pK 2.148 pK7.198 pK.375 You wish to prepare 1.000 L of a 0.0100 M phosphate buffer at pH 7.440. To do this, you choose to 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? Number Number Nall, PO,- mass Na,HPO- Na: HPO,-10 mass mass What other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer? (Check all that apply). HPO4 and NaH2PO4 HyPO4 and Na2HPO4 HPO4 and NayPO4 NaH2P04 and Na3PO4 Na2HPO4 and Na3PO4Explanation / Answer
pKa1 = 2.148 = [H+][H2PO4 -] / [H3PO4]
2.148 = x^2 / 0.01 - x
x^2 + 2.148x - 0.02148 = 0
x = [H2PO4-] = 0.00995M
pH = pKa + log[Na2HPO4] / [NaH2PO4]
7.44 = 7.198 + log[Na2HPO4] / [NaH2PO4]
0.242 = log[Na2HPO4] / [NaH2PO4]
1.745 = [Na2HPO4]/[NaH2PO4]
you will need 1.745 x the amount of Na2HPO4 as NaH2PO4
x / y = 1.745
x + y = 0.01
1.745y + y = 0.01
2.745y = 0.01
y = 0.003M = [NaH2PO4]
x = 0.007M = [Na2HPO4]
0.0036moles NaH2PO4 x 120g/mole = 0.432g
0.007moles Na2HPO4 x 139g/mole = 0.973g
H3PO4 and Na3HPO4