Using the K_a\'s for HC_2H_3O_2 and HCO_3^- (from Appendix F), calculate the K_b

Question

Using the K_a's for HC_2H_3O_2 and HCO_3^- (from Appendix F), calculate the K_b's for the C_2H_3O_2^- and CO_3^2- ions. Compare these values with those calculated from your measured pH's. Using K_b for NH_3 (from Appendix G), calculate K_a for the NH_4^+ ion. Compare this value with that calculated from your measured pH's How should the pH of a 0.1 M solution of NaC_2H_3O_2 compare with that of a 0.1 M solution of KC_2H_3O_2? Explain briefly. What is the greatest source of error in this experiment? How could you minimize this source of error?

Explanation / Answer

(1) & (2):

The answer for both these parts are similar.

Use the following relation:

Kw = Ka*Kb

Here, Kw = ionic product constant for water = 10-14 at 250C

Switch in the Ka value for the acid and calculate the Kb value for the conjugate base using this relation. Alternatively, switch in the Kb value for the base and calculate the Ka for the conjugate acid.

(3)

The pH values for these two solutions will be similar. This is because both NaOH and KOH are considered strong bases. So both dissociate completely.

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