Phosphoric acid is triprotic. Ka1 = 7.5 x10-3, Ka2=6.2x10-8 and Ka3=4.2x10-13. p
ID: 565170 • Letter: P
Question
Phosphoric acid is triprotic. Ka1 = 7.5 x10-3, Ka2=6.2x10-8 and Ka3=4.2x10-13.
pH
Indicator
1
2
3
4
5
6
7
8
9
10
11
Phenolphthalein
Colorless
Pink
Red
Methyl Red
Red
Orange
Yellow
Orange IV
Orange
Peach
e) What would be the pH of a solution prepared by combining equal quantities of sodium dihydrogen phosphate and sodium hydrogen phosphate?
f) Sufficient strong acid is added to a solution containing Na2HPO4 to neutralize one half of it. What will be the pH of this solution? Explain.
pH
Indicator
1
2
3
4
5
6
7
8
9
10
11
Phenolphthalein
Colorless
Pink
Red
Methyl Red
Red
Orange
Yellow
Orange IV
Orange
Peach
Explanation / Answer
H3PO4 phosphoric acid
e) when equal amount of NaH2PO4 and NaH2PO4 are mixed together
according to Hendersen-Hasselbalck equation,
pH = pKa2 + log([Na2HPO4]/[NaH2PO4])
with,
[NaH2PO4] = [NaH2PO4]
pH = pKa2
= -log(Ka2)
= -log(6.2 x 10^-8)
= 7.20
--
f) when strong acid added to neutralize 1/2 of Na2HPO4
we will have,
[HPO4^2-] remained = [H2PO4^-] formed
in solution.
according to Hendersen-Hasselbalck equation,
pH = pKa2+ log([HPO4^2]/[H2PO4^-])
with,
pH = pKa2
= -log(Ka2)
= -log(6.2 x 10^-8)
= 7.20