Titrations: Diprotic & Polyprotic Buffers Practice Problems

Consider the titration of selenic acid (H₂SeO₄), a diprotic acid, with an aqueous KOH solution. The diagrams below depict different stages of titration. For simplicity, K⁺ ions and water molecules are not shown. 

Identify the solution with the highest pH and show the initial state of the solution before the addition of KOH.

Consider the titration of selenic acid (H₂SeO₄), a diprotic acid, with an aqueous KOH solution. The diagrams below depict different stages of titration. For simplicity, K⁺ ions and water molecules are not shown. 

Match each diagram to the correct stages of titration: 

(i) First half-equivalence point 

(ii) Second half-equivalence point

(iii) Second equivalence point 

(iv) After the second equivalence point

The curve below shows the titration of a diprotic acid H₂A with NaOH. Estimate the pKa values for the diprotic acid.

Titration of 75.0 mL of a 1.25 M solution of citric acid with 37.5 mL of 1.25 M NaOH was done. Calculate the pH if the pK_a values for the first, second, and third dissociations are 3.13, 4.76, and 6.40 respectively.

75.0 mL of 1.50 M H₃AsO₄ is titrated with 1.50 M NaOH. What is the pH for the titration after 150.0 mL of base is added? (H₃AsO₄; K_a1 = 5.5×10⁻³, K_a2 = 1.7×10⁻⁷, K_a3 = 5.1×10⁻¹²)

A 75.0 mL of a 0.125 M solution of the protonated form of glycine was titrated with 112.5 mL of 0.125 M NaOH. Find the pH if the pK_a values for the first and second dissociations are 2.34 and 9.78 respectively.

What is the pH for the titration of 55.0 mL of a 0.15 M solution of the protonated form of valine (H₂A⁺: K_a1 = 5.13×10⁻³, K_a2 = 1.82×10⁻¹⁰) with 110.0 mL of 0.15 M KOH?

When a 15.0 mL sample of 5.45 g/L solution of a diprotic acid was titrated against 0.15 M KOH, it required 24.50 mL of the base to reach the endpoint. The pH values at the first and second equivalence points were 5.75 and 10.75, respectively. Calculate the pKa1, pKa2, and the molecular weight of the acid.

The diagrams below show the different stages of titration for a weak diprotic acid H₂X(aq) with KOH(aq). For simplicity, water molecules and K⁺ ions are not shown. Identify the solution that has the highest pH and the solution that has the lowest pH.

In a laboratory experiment, you need to prepare a solution buffered at pH = 6.00. The organism that you are working with is not sensitive to the weak acid malonic acid (H₂C₃H₂O₄: K_a1 = 1.5×10^–3; K_a2 = 2.0×10^–6) and its sodium salts. Solutions of 1.5 M malonic acid and 1.5 M NaOH are available in the laboratory. Calculate the volume of NaOH solution that should be added to the 1.5 L of malonic acid in order to create a buffer solution buffered at pH = 6.00. Assume that volume does not change.

One of the following graphs represents the titration of a triprotic acid with a strong base. Which titration curve is it?

17.2 mL of 0.212 M HCl was neutralized with 29.3 mL of a KOH solution. The same KOH solution was used to titrate 15.0 mL of a H₃AsO₄ solution. 38.2 mL of the KOH solution was used to neutralize the H₃AsO₄ solution. What is the molarity of the H₃AsO₄ solution?

What is the molar mass of a 0.738 g of unknown diprotic acid neutralized by titrating with 71.24 mL of 0.167 M calcium hydroxide?

In the titration of 15.0 mL of 0.113 M carbonic acid (H₂CO₃) with 0.129 M NaOH, how much volume of the base is needed to reach each equivalence point?

Phosphoric acid (H₃PO₄) and (NaOH) react according to the following equation:

H₃PO₄(aq) + NaOH(aq) → H₂O(l) + Na₃PO₄(aq)       [unbalanced]

A 0.150 M NaOH solution was used to titrate a 20.00 mL H₃PO₄ solution with an unknown concentration. If 21.54 mL of NaOH solution was used to reach the equivalence point, determine the concentration of the H₃PO₄ solution.

A 0.385 g sample of weak diprotic acid is titrated against 0.220 M NaOH. 38.8 mL of NaOH is required to reach the endpoint of the titration. Calculate the molar mass (in g/mol) of the acid.

The titration curve below shows a weak diprotic acid (H₂X) being titrated against NaOH. Identify which of the point (A—D) on the curve, show the buffer region where [H₂X] = [HX^-]?

A diamine is a compound that contains two amino groups and can accept two protons since each nitrogen in the compound can accept a proton. Propylenediamine (1-2-Diaminopropane), H₂NC₃H₆NH₂, is an example of a diamine:

H₂NC₃H₆NH₂(aq) + H₂O(l) ⇌ H₃NC₃H₆NH₂⁺(aq) + OH⁻(aq)          K_b1 = 6.61×10⁻⁵

H₃NC₃H₆NH₂⁺(aq) + H₂O(l) ⇌ H₃NC₃H₆NH₃²⁺(aq) + OH⁻(aq)       K_b2 = 4.07×10⁻⁸

Determine the pH of the solution after the addition of the following volumes of 0.300 M HCl to 30.0 mL of 0.200 M propylenediamine.

(i) 0 mL

(ii) 10.0 mL

(iii) 20.0 mL

(iv) 30.0 mL

(v) 40.0 mL

(vi) 50.0 mL

Neutralization reactions involving a strong acid like HCl proceeds to completion. A 4.05 g sample of Na₃C₆H₅O₇ was dissolved in 275 mL of 0.160 M HCl. Write the balanced net ionic equations for the neutralization reactions involved.  Find the pH of the resulting solution. The K_a for citric acid is 7.41×10⁻⁴.