20.4 The pH of Weak Acids

Question 1

Dissociation of weak acids?

Answer 1

[H+] =/= [HA]

a weak acid partially dissociates

Question 2

What two things can vary [H+]?

Answer 2

~Concentration of acid [HA(aq)]

~Acid Dissociation constant Ka

Question 3

Approximations for weak acids

Answer 3

1) Dissociation of water is negligible
[H+(aq)]eqm = [A-(aq)] eqm
2). Concentration of acid is much greater than the [H+] at eqm
[HA(aq)]start >> [H+(aq)]eqm
[HA(aq)]eqm = [HA(aq)]start

Question 4

Simplify the Ka expression

Answer 4

Ka = ([H+(aq)]eqm^2)

([HA(aq)]start)

Question 5

Two ways of determining Ka experimentally

Answer 5

~Preparing a standard solution of the weak acid of known concentration
~Measuring the pH of the standard solution using a pH meter