Acids, Bases and Buffers

higher Ka

stronger acid

pKa

-logKa

pH

=-log₁₀[H⁺]

[H⁺]

= 10^pH

Kw

= [H⁺(aq)][OH⁻(aq)]

Ka

= [H⁺]²/[HA]

Assumption 1 for weak acids

the dissociation of water is negligable

Assumption 1 breaks down…

for very weak acids or very dilute solutions

Assumption 2

[HA] at equilibrium is the same as at the start

Assumption 2 breaks down…

for “stronger” weak acids or very dilute solutions

A buffer solution is

a system that minimises pH changes when a small amounts of acid or base are added

If H⁺ is added to a buffer:

1. [H⁺] increases

2. H⁺ ions react with the conjugate base, A⁻

3. The equilibrium shifts to the left to restore [H⁺]

If OH⁻ is added to the buffer:

1. [OH⁻] increases

2. The small H⁺ ion conc. reacts with the OH⁻ ions to make water

3. Equilibrium shifts to the right to restore [H⁺]

Indicator for strong acid-strong base

phenolphthalein or methyl orange

indicator for weak acid-strong base

phenolphthalein

indicator for strong acid-weak base

methyl orange

indicator for weak acid-weak base

none

lower pKa

stronger acid

value of Kw =

1 × 10⁻¹⁴