[20.4] the pH of weak acids

Question 1

what is a weak acid?

Answer 1

an acid that partially dissociates in an aqueous solution

Question 2

what does [H+(aq)] depend on in the weak acid equilibrium?

Answer 2

• concentration of the acid [HA(aq)[
• acid dissociation constant, Ka

Question 3

calculating Ka using equilibrium concentrations

Answer 3

[H+] eqm [A-] eqm / [HA] eqm = [ H+] eqm [A-] eqm / [HA] start - [H+] eqm

Question 4

assumptions: [H+] = [A-]

Answer 4

• there is source of H+ from water itself when it dissociates into H+ and OH-
• we can ignore the ionisation of water because water produces far less H+ than most weak acids

Question 5

assumptions: amount of HA at equilibrium is equal to the amount of HA put into the solution

Answer 5

• we can ignore the fraction of HA that has been convered to H+
• we can do this as for weak acids, this reaction is very small

Question 6

when are weak acid assumptions not justified?

Answer 6

• stronger weak acids with Ka > 10⁻²mol dm⁻³
• very dilute solutions

Question 7

equation for Ka for all solutions containing a weak acid

Answer 7

Ka = [H+] [A-] / [HA]

not used when calculating pH of weak acids

Question 8

equation for Ka of weak acid in water only

Answer 8

Ka = [H+]² / [HA]