For a conjugated acid- base pairs, how are Ka and Kb related? Consider the react

Question

For a conjugated acid- base pairs, how are Ka and Kb related? Consider the reaction of acetic aid in water CH3Co2H (aq) + H2O (l) --> <--- CH3CO2- (aq) + H3O+ (aq) where Ka= 1.8 x 10^ -5.

a. Which two bases are comepting for the proton?
b. Which is the stronger base?
c. In light of your answer to part b, why do we classify the acetate ion (CH3CO2-) as a weak base? Use an appropriate reaction to justify your answer.

In general, as base strength increases conjugate acid strength decreases. Explain why the conjugate acid of the weak base NH3 is a weak acid.

To summarize, the conjugate base of a weak acid is a weak base and the conjugate acid of a weak basse is a weak acid (weak gives you weak). Assuming Ka for a monoprotic strong acid is 1

Explanation / Answer

Relation between Ka and Kb :

Kw = Ka * Kb where Kw = ionization constant of water = 10-14 . Thus, Ka* Kb = 10-14

a) The two bases which can compete for a proton are:

CH3COO- to form CH3COOH

H2O to form H3O+

b) CH3COO- is the conjugate base of CH3COOH which is a weak acid. Hence, CH3COO- is a stronger base than water

c) Now, Kb of CH3COO- = 10^-14/Ka = 10^-14/1.8*10^-5 = 5.56 * 10^-10. this lies inbetween the base strength of H2O = 10^-14 and that of the strongest base OH- = 1. As a result, CH3COO- is classified as a base which is stronger than water but weaker than OH-.

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