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Solutions of Polyprotic Acid/Base Systems, Problem C

3. Calculate the pH of a solution prepared by adding 30 mL of 0.1 M hydrochloric acid to 60 mL of 0.080 M potassium malonate.

What sort of reaction is occurring?

After several minutes the groups should recognize that the malonate can be protonated as it is neutralized by the strong acid.

Since a neutralization reaction is occurring, what is Kn?

Students should immediately recognize that since HCl is a strong acid Kn will be large and the reaction can be assumed to go to completion.

What is in the solution after the neutralization occurs?

After several minutes, the students should recognize that once the HCl has converted some of the malonate, a buffer is formed. Make sure that they convert the amounts into moles since dilution will occur. They may be tempted to incorporate the fully protonated malonic acid into the equilibrium but remind them that the Ka values differ so much that formation of malonic acid is negligible.

Calculate the concentration of malonic acid at equilibrium to ensure that it is negligible.

Which Ka value should be used in the Henderson-Hasselbalch equation?

Students should recognize that since the K value that relates the two species forming the buffer is Ka2, they should use that value in the Henderson-Hasselbalch equation.

What is the pH of the solution?