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AQA GCSE Chemistry Strong and weak acids

Strong and weak acids

This page covers the following topics:

1. Strong acids
2. Weak acids
3. pH and concentration
4. Acid terms

Acids are solutions which act as a source of H⁺ ions. To be classified as a strong acid, the substance must completely dissociate into its ions in solution, represented by a single arrow in the reaction equation. Strong acids can be identified by their very low pH, usually below pH 1.

Strong acids

Weak acids only partially dissociate into its ions in solution, represented by a equilibrium arrow. Weak acids can be identified by their pH, usually between pH 3-5. Carboxylic acids are a great example of weak acids.

Weak acids

pH is a measure of the concentration of protons in a solution, shown in the equation below. The logarithmic relationship between pH and [H⁺] means that a change in proton concentration by a factor of 10 is equivalent to a change in pH of 1.

pH and concentration

There are many key terms used to describe an acid. An acid can be dilute or concentrated, describing the concentration of an acid in water. An acid can also be strong or weak, describing the ability of an acid to donate protons. This is distinguished using the acids pH.

Acid terms

1

What pH range do weak acids usually fall within?

What pH range do weak acids usually fall within?

2

Identify the conjugate base in the following equation:

HNO₃ + Hβ‚‚O β‡Œ H₃O⁺ + NO₂⁻.

Identify the conjugate base in the following equation: 

HNO₃ + Hβ‚‚O β‡Œ H₃O⁺ + NO₂⁻.

3

A basic solution has a proton concentration of 2.5 x 10⁻¹⁰ M. What is the pH of this solution?

A basic solution has a proton concentration of 2.5 x 10⁻¹⁰ M. What is the pH of this solution?

4

What is the defining property of a strong acid?

What is the defining property of a strong acid?

5

Calculate the pH of 1.5 M sulphuric acid to 2 decimal places.

Calculate the pH of 1.5 M sulphuric acid to 2 decimal places.

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