Polyprotic Acid Example Chemistry Problem

How to Work a Polyprotic Acid Problem

Sulfuric acid is an example of a common polyprotic acid.
Sulfuric acid is an example of a common polyprotic acid. LAGUNA DESIGN / Getty Images
By
Anne Marie Helmenstine, Ph.D.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
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Updated on February 04, 2019

A polyprotic acid is an acid that can donate more than one hydrogen atom (proton) in an aqueous solution. To find the pH of this type of acid, it's necessary to know the dissociation constants for each hydrogen atom. This is an example of how to work a polyprotic acid chemistry problem.

Polyprotic Acid Chemistry Problem

Determine the pH of a 0.10 M solution of H2SO4.

Given: Ka2 = 1.3 x 10-2

Solution

H2SO4 has two H+ (protons), so it is a diprotic acid that undergoes two sequential ionizations in water:

First ionization: H2SO4(aq) → H+(aq) + HSO4-(aq)

Sulfuric acid is a polyprotic acid, able to donate two hydrogen ions to aqueous solution.
Read More
Polyprotic Acid Definition in Chemistry
By Anne Marie Helmenstine, Ph.D.

Second ionization: HSO4-(aq) ⇔ H+(aq) + SO42-(aq)

Note that sulfuric acid is a strong acid, so its first dissociation approaches 100%. This is why the reaction is written using → rather than ⇔. The HSO4-(aq) in the second ionization is a weak acid, so the H+ is in equilibrium with its conjugate base.

Ka2 = [H+][SO42-]/[HSO4-]

Ka2 = 1.3 x 10-2

Ka2 = (0.10 + x)(x)/(0.10 - x)

Since Ka2 is relatively large, it's necessary to use the quadratic formula to solve for x:

x2 + 0.11x - 0.0013 = 0

x = 1.1 x 10-2 M

The sum of the first and second ionizations gives the total [H+] at equilibrium.

0.10 + 0.011 = 0.11 M

pH = -log[H+] = 0.96

Learn More

Introduction to Polyprotic Acids

Strength of Acids and Bases

Concentration of Chemical Species

First Ionization H2SO4(aq) H+(aq) HSO4-(aq)
Initial 0.10 M 0.00 M 0.00 M
Change -0.10 M +0.10 M +0.10 M
Final 0.00 M 0.10 M 0.10 M
Second Ionization HSO42-(aq) H+(aq) SO42-(aq)
Initial 0.10 M 0.10 M 0.00 M
Change -x M +x M +x M
At Equilibrium (0.10 - x) M (0.10 + x) M x M
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Helmenstine, Anne Marie, Ph.D. "Polyprotic Acid Example Chemistry Problem." ThoughtCo, Jul. 29, 2021, thoughtco.com/polyprotic-acid-example-problem-603633. Helmenstine, Anne Marie, Ph.D. (2021, July 29). Polyprotic Acid Example Chemistry Problem. Retrieved from https://www.thoughtco.com/polyprotic-acid-example-problem-603633 Helmenstine, Anne Marie, Ph.D. "Polyprotic Acid Example Chemistry Problem." ThoughtCo. https://www.thoughtco.com/polyprotic-acid-example-problem-603633 (accessed April 23, 2024).