Question

In the lecture we used titration of sodium carbonate with hydrochloric acid as an example to explain how to titrate a weak base with a strong acid. Suppose that you are using HNO3 to titrate Na3PO4, describe how to calculate pH at following different titration stages: (a) before titration (b) at 19 equivalence point (c) at 3rd equivalence point (d) between 2nd and 3rd equivalence points (e) after 3rd equivalence point Calculation is not required, simply write the formulas. For example: treat as monoprotic acid, [H+] = K F Even though we didn't talk about titrating weak acid with strong base, the idea is similar. Assume that 22.00 mL of NaOH (0.100 M) was consumed to titrate a 10.00 mL H2CO3 solution (fully converted to Na2CO3) pKai = 6.351 and PK 2 = 10.329. (a) what is the concentration of the initial H2CO3 solution? (b) what is the pH of the H2CO3 solution before titration? (c) which is the principal (dominating) species when 5.00 mL of NaOH has been added? (d) what is the pH of the solution when 15.00 mL of NaOH has been added? (e) what are the two most abundant species at pH=10.329?

thank you

Answer 1

Question 1.

(a) [H⁺] = (Kw*Ka3/[Na3PO4])^1/2, pH = -Log[H⁺]

(b) [H⁺] = (Ka2*Ka3)^1/2, pH = -Log[H⁺]

(c) [H⁺] = ([H3PO4]*Ka1)^1/2, pH = -Log[H⁺]

(d) [H⁺] = Ka1*[H3PO4]/[NaH2PO4], pH = -Log[H⁺]

(e) [H⁺]_total = [H⁺]_H3PO4+ [H⁺]_HNO3, pH = -Log[H⁺]

Note: [H⁺]_H3PO4= ([H3PO4]*Ka1)^1/2; [H⁺]_HNO3 = [HNO3]