Acid Dissociation Constant

Question 1

What happens when a weak acid dissociates in aqueous solutions?

Answer 1

• Only partly dissociates
• Equilibrium shifts left (more of the weak acid than the H+ and A- ions)

Question 2

What is the acid dissociation constant?

Answer 2

Ka = [H+] [A-] / [HA] moldm^-3

Question 3

Acid dissociation notes

Answer 3

• Ka often very small
• Assumption made that [H+] is negligible because of ionisation of water
• Higher value for Ka, more dissociated and stronger it is

Question 4

How to calculated pH of weak acids

Answer 4

If we know :
- Concentration of acid
- Ka value of acid

Equilibrium concentration of [H+] and [A-] will be same since one molecule of HA dissociates into 1 of each ion, so expression can be written as

Question 5

What is pKa?

Answer 5

pKa = -logKa

Question 6

How can Ka values be found when a weak acid reacts with a strong base?

Answer 6

• When half acid is neutralised, [HA] = [A-]
• So Ka = [H+]

In practice, neutralised by strong base and pH titration curve plotted