How do you calculate buffer pH for monoprotic acids?

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Answer 1 · 2015-04-30T21:43:40

You use the Henderson-Hasselbalch equation.

A monoprotic acid has one ionizable hydrogen.

#"HA" + "H"_2"O" ⇌ "H"_3"O"^+ + "A"^-#

A solution that contains both #"HA"# and #"A"^-# is a buffer solution, and its pH is given by the Henderson-Hasselbalch equation.

#"pH" = "p"K_"a" + log((["A"^-])/(["HA"]))#

EXAMPLE

Calculate the pH of a solution that contains 0.20 mol/L acetic acid and 0.10 mol/L sodium acetate. The #K_"a"# for acetic acid is #1.74 × 10^-5#.

Solution

#"pH" = "p"K_"a" + log ((["A"^-])/(["HA"])) = -log(1.74 × 10^-5 ) + log((0.10 cancel("mol/L"))/(0.20 cancel("mol/L"))) = 4.76 – 0.30 = 4.46#