A weak acid is one that is only partially ionised in aqueous solution, so an equilibrium is set up between the acid and its ions.

pKa is the negative logarithm to base ten of the acid dissociation constant.

State the two assumptions made when the pH of a weak acid is calculated from Ka.

The hydrogen ion concentration is taken as equal to the anion concentration, and the concentration of undissociated acid is taken as equal to the concentration made up.

Why is the ionisation of water ignored in a weak acid pH calculation?

The acid supplies far more hydrogen ions than the water does, so the contribution from water is negligible in comparison.

CCEA A-Level Chemistry 1110

Formulae, Equations and Amounts of Substance

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Acid-Base Titrations

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Equilibrium and Acid-Base Equilibria

Ka, pKa and the pH of weak acids

3 things to know
Unit A2 1

Overview

Weak acid and units of Ka for a weak monobasic acid.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Define a weak acid.

Key term

Write the expression for the acid dissociation constant of a weak monobasic acid HA.

Key term

State the units of Ka for a weak monobasic acid.

Questions from this subtopic

Each one with the model answer underneath.

1
Define a weak acid.
Model answer
A weak acid is one that is only partially ionised in aqueous solution, so an equilibrium is set up between the acid and its ions.
2
Write the expression for the acid dissociation constant of a weak monobasic acid HA.
Model answer
The expression is Ka = [H⁺][A⁻] ÷ [HA].
3
State the units of Ka for a weak monobasic acid.
Model answer
The units are mol dm⁻³.
4
Define pKa.
Model answer
pKa is the negative logarithm to base ten of the acid dissociation constant.
5
State the two assumptions made when the pH of a weak acid is calculated from Ka.
Model answer
The hydrogen ion concentration is taken as equal to the anion concentration, and the concentration of undissociated acid is taken as equal to the concentration made up.
6
Why is the ionisation of water ignored in a weak acid pH calculation?
Model answer
The acid supplies far more hydrogen ions than the water does, so the contribution from water is negligible in comparison.

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Ka, pKa and the pH of weak acids

Overview

What you need to know

Define a weak acid.

Write the expression for the acid dissociation constant of a weak monobasic acid HA.

State the units of Ka for a weak monobasic acid.

Questions from this subtopic

Define a weak acid.

Write the expression for the acid dissociation constant of a weak monobasic acid HA.

State the units of Ka for a weak monobasic acid.

Define pKa.

State the two assumptions made when the pH of a weak acid is calculated from Ka.

Why is the ionisation of water ignored in a weak acid pH calculation?

Other ways to work this

What is inside Levo

The 3-step method, run for you

Cram a whole paper in one sitting

Blank Blitz makes facts automatic

Build It gets the exam shape right

Test It catches misconceptions

Brain Breaks between sessions

Every question in Equilibrium and Acid-Base Equilibria, on a schedule.