Propionic acid is a weak organic acid. A 0.123 M propionic solution is titrated
ID: 509921 • Letter: P
Question
Propionic acid is a weak organic acid. A 0.123 M propionic solution is titrated by a 0.123 M sodium hydroxide solution. The initial volume of the propionic solution is 65.0 mL.
The Ka of propionic acid is 1.34 x 10-5.
A. Calculate the pH before any sodium hydroxide has been added (initial solution)
B. Calculate the pH after 20.0 mL of sodium hydroxide has been added.
C. Calculate the pH after 50.0 mL of sodium hydroxide has been added.
D. Calculate the pH at the equivalence point
E. Calculate the pH after 80.0 mL of sodium hydroxide has been added.
Explanation / Answer
A)
Ka = 1.34 x 10^-5
pKa = 4.87
pH = 1/2 (pKa - log C)
= 1/2 (4.87 - log 0.123)
= 2.89
pH = 2.89
B)
millimoles of propanoic acid = 0.123 x 65 = 7.995
millimoles of NaOH = 20 x 0.123 = 2.46
acid + NaOH ---------------> salt + H2O
7.995 2.46 0
5.535 0 2.46
pH = pKa + log [salt / acid]
= 4.87 + log [2.46 / 5.535]
= 4.52
pH = 4.52
C)
millimoles of NaOH = 50 x 0.123 = 6.15
acid + NaOH ---------------> salt + H2O
7.995 6.15 0
1.845 0 6.15
pH = pKa + log [salt / acid]
= 4.87 + log [6.15 / 1.845]
= 5.39
pH = 5.39
D)
salt concentration = 7.995 / (65 +65) = 0.0615 M
pH = 7 + 1/2 (pKa + log C)
= 7 + 1/2 (4.87 + log 0.0615)
= 8.83
pH = 8.83
E)
[OH-] = 0.0127 M
pOH = 1.90
pH = 12.10
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