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Comparing concentration with strength

I can use the pH scale to work out concentration differences between solutions relating to their pH, and discuss the differences between strong and weak acids.

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Year 10
Edexcel
Higher

Comparing concentration with strength

I can use the pH scale to work out concentration differences between solutions relating to their pH, and discuss the differences between strong and weak acids.

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Lesson details

Key learning points

  1. pH is a logarithmic scale (as hydrogen ion concentration increases by a factor of 10, the pH decreases by 1).
  2. For a given concentration, a stronger acid contains more H⁺ ions.
  3. Titration curves can be used to show the change in pH to allow chemists to pinpoint a more accurate end point.
  4. A strong acid is completely ionised in aqueous solution, a weak acid only partially ionises.
  5. Titration curves show how the pH changes throughout a neutralisation reaction.

Keywords

  • Hydrogen ion - A positively charged ion formed when a hydrogen atom loses an electron (H⁺).

  • PH - The measure of the hydrogen ion (H⁺) concentration of a solution.

  • Concentration - The amount of solute dissolved in a certain volume of solution.

  • Strong acid - An acid which fully dissociates to form H⁺ ions in aqueous solution.

  • Weak acid - An acid which partially dissociates to form H⁺ ions in aqueous solution.

Common misconception

Pupils often confuse the chemistry meaning of strong/weak and concentrated/dilute due to the use of these words in everyday conversation.

Clear definitions of the words are required with practice opportunities to recognise/describe whether the substances are strong or weak acids and if concentrated or dilute.

pH provides a good opportunity to practise magnitude calculations where changes in pH are a reflection of changes in the magnitude of H⁺ ion concentration. Practise by giving either the pH value or concentration of a solution and then ask pupils to calculate the amended value when a change occurs.
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This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.
What is the pH scale used to measure?
the concentration of salts in a solution
the colour of a solution
Correct answer: the acidity or alkalinity of a solution
the temperature of a solution
Q2.
What is ionisation?
the process of dissolving a substance in water
the release of energy in a reaction
the mixing of acids and bases
Correct answer: the formation of ions by gaining or losing electrons
Q3.
What is the charge of a hydrogen ion?
negative
neutral
Correct answer: positive
Q4.
What does it mean if an acid is completely ionised in an aqueous solution?
the acid has fully reacted with a base
Correct answer: all acid molecules dissociate into ions
the acid becomes neutral
the acid stops reacting
Q5.
What does a weak acid do in an aqueous solution?
it completely dissociates into ions
Correct answer: it partially dissociates into ions
it does not ionise
it becomes a strong base
Q6.
What is the significance of titration curves?
they show the temperature change during a reaction
they indicate changes in volume
Correct answer: they graph pH changes to find neutralisation points
they track time during a reaction

Exit quiz

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6 Questions

Q1.
Match the following key terms to their definitions.
Correct Answer:hydrogen ion (H⁺),a positive ion formed when a hydrogen atom loses an electron

a positive ion formed when a hydrogen atom loses an electron

Correct Answer:pH,the measure of hydrogen ion concentration in a solution

the measure of hydrogen ion concentration in a solution

Correct Answer:concentration,the amount of solute dissolved in a certain volume of solution

the amount of solute dissolved in a certain volume of solution

Correct Answer:strong acid,an acid that fully dissociates to form H⁺ ions in aqueous solution

an acid that fully dissociates to form H⁺ ions in aqueous solution

Correct Answer:weak acid,an acid that partially dissociates to form H⁺ ions in aqueous solution

an acid that partially dissociates to form H⁺ ions in aqueous solution

Q2.
Which of the following statements about a strong acid in an aqueous solution is correct?
It is partially ionised.
It releases very few H⁺ ions.
Correct answer: It is completely ionised.
It has a pH above 7.
Q3.
Why do strong acids have a lower pH than weak acids of the same concentration?
Correct answer: they contain more H⁺ ions
they contain fewer H⁺ ions
they ionise partially
they absorb more water
Q4.
What happens to the pH of a solution if the hydrogen ion concentration increases by a factor of 10?
the pH increases by 1
Correct answer: the pH decreases by 1
the pH increases by 10
the pH decreases by 10
Q5.
How can titration curves help chemists?
they measure the temperature of a reaction
they show the colour change in the solution
Correct answer: they allow chemists to pinpoint an accurate end point
they help identify weak acids
Q6.
What happens during the neutralisation of a weak acid?
the pH decreases rapidly at first
the pH stays constant throughout
Correct answer: the pH changes gradually at the start
the pH changes quickly after the end point is reached