Phosphoric acid is a triprotic acid with the following pKa values: pKa1=2.148 pK
ID: 896669 • Letter: P
Question
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 7.720. 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?
mass NaH2PO4= ________g mass Na2HPO4= __________g
What other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer? (Check all that apply).
H3PO4 and NaH2PO4
H3PO4 AND Na2HPO4
H3PO4 and Na3PO4
NaH2PO4 and Na3PO4
Na2HPO4 and Na3PO4
Explanation / Answer
let the number of moles of NaH2PO4 be x.so, number of moles if Na2HPO4=0.02-x
so,
pH=pKa + log(salt/acid)
7.72 = 7.198 + log((0.02-x)/x)
or x=4.622*10^-3
so mass of Na2HPO4=142*(0.02-4.622*10^-3)
=2.1836 grams
mass of NaH2PO4=120*4.622*10^-3
=0.554 grams
the same buffer can be prepared by Na2HPO4 and Na3PO4