Weak acids and bases Ka for weak acids (3.1.12.4)

What are weak acids and bases?

Weak acids and bases only slightly dissociate in aqueous solution to form ions

What are strong acids and bases?

Strong acids and bases dissociate fully in aqueous solution to form ions

What is K_a and what are the equations for it?

K_a is the acid dissociation constant

Where HA is the weak acid:

K_a = [H⁺][A^-] / [HA]

pK_a = -log K_a

K_a = 10^-pK_a

The previous expressions hold for weak acids at all times

K_a has units moldm^-3

Either pK_a or K_a are usually given in the question

What are the equations that hold when a weak acid is in water with nothing else added?

[H⁺] = [OH^-]

[HA] = [HA]_initial

K_a = [H⁺]²/[HA]

These equations can be used to find the pH of weak acids

What is a half neutralisation reaction?

A half neutralisation reaction is when a weak acid reacts with a strong base, and half the weak acid reacts with the base which means [HA] = [A^-]. Therefore pK_a = pH (K_a = [H⁺])

How can the pH of a solution where a weak acid has reacted with a strong base be found (if the HA is in excess or if the OH^- is in excess)?

To find which is in excess

1) Calculate n(HA)

2) Calculate n(OH^-)

3) Calculate excess using an ice table

If HA is in excess

1) Calculate n(HA) left and n(A^-) formed

2) Calculate [HA] left and [A^-] formed

3) Use K_a to find [H⁺]

4) Find pH

If OH^- is in excess

1) Calculate [OH^-]

2) Use K_w to find [H⁺]

3) Find pH