Question 1 Medium
Write the equation for hydrochloric acid ionising in water.
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Answer: HCl → H⁺ + Cl⁻
- It is completely ionised.
- HCl → H⁺ + Cl⁻.
pH, strong acids and weak acids: GCSE Combined Science Chemistry practice questions
A second set of questions on pH, strong acids and weak acids. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
More questions
Question 2 Medium
What arrow is used in the equation for a weak acid ionising?
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Answer: ⇌
- A weak acid is partly ionised.
- The reaction is reversible.
Question 3 Medium
Nitric acid and citric acid both have a concentration of 1 mol/dm³. Which has the lower pH?
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Answer: nitric acid
- Nitric acid is a strong acid, so it is completely ionised.
- It makes more hydrogen ions than citric acid, so its pH is lower.
Question 4 Medium
Which solution has the higher concentration of hydrogen ions, one with pH 2 or one with pH 5?
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Answer: pH 2
- A lower pH means more hydrogen ions.
- So the solution with pH 2 has more.
Question 5 Hard
Explain why 1.0 mol/dm³ ethanoic acid has a higher pH than 1.0 mol/dm³ hydrochloric acid.
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Answer: it is only partly ionised so it has fewer hydrogen ions
- Ethanoic acid is only partly ionised.
- It makes fewer hydrogen ions, so the pH is higher.
Question 6 Hard
Solution P has a pH of 6 and solution Q has a pH of 2. How many times greater is the concentration of hydrogen ions in Q?
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Answer: 10000
- The difference is 4 units.
- 10⁴ = 10 000.
Question 7 Hard
Magnesium is added to equal volumes of 1 mol/dm³ hydrochloric acid and 1 mol/dm³ ethanoic acid. Which acid makes bubbles faster, and why?
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Answer: hydrochloric acid, because it has more hydrogen ions
- Hydrochloric acid has a higher concentration of hydrogen ions.
- So it reacts faster.
Question 8 Hard
A student says a dilute acid must be a weak acid. Give one reason why the student is wrong.
Show answer
Answer: strength is about ionisation, not concentration
- Dilute is about concentration.
- Strong is about ionisation. A dilute acid can be strong.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What does an acid release in water?
Show answer
Hydrogen ions, H⁺.
Check 2
What is a strong acid?
Show answer
One that is completely ionised in water.
Check 3
What is a weak acid?
Show answer
One that is only partly ionised in water.
Check 4
Name three strong acids.
Show answer
Hydrochloric, sulfuric and nitric acid.
Check 5
Name three weak acids.
Show answer
Ethanoic, citric and carbonic acid.
Check 6
What arrow shows a weak acid ionising?
Show answer
The reversible arrow ⇌.
Check 7
What does concentration mean?
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The amount of acid dissolved in a given volume.
Check 8
Can a dilute acid be strong?
Show answer
Yes. Strength and concentration are different.
Check 9
What does pH measure?
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The concentration of hydrogen ions.
Check 10
What happens to hydrogen ion concentration when pH falls by 1?
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It goes up tenfold.
Check 11
How many times more hydrogen ions at pH 2 than pH 4?
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100 times.
Check 12
What ion do alkalis release?
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Hydroxide ions, OH⁻.
Check 13
Which has the lower pH at equal concentration, a strong or weak acid?
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The strong acid.
Check 14
Why does a weak acid react more slowly?
Show answer
It has fewer hydrogen ions.
Check 15
What is ionisation?
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The splitting of a substance into ions when it dissolves.
Check 16
What does completely ionised mean for an acid?
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Every molecule of the acid splits into ions.
Check 17
What does a reversible arrow tell you about an acid?
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It is only partly ionised, so the change can go both ways.
Check 18
What is the symbol for a hydrogen ion?
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H⁺.
Check 19
What is the pH of a neutral solution?
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7.
Check 20
Is a solution with pH 3 acidic or alkaline?
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Acidic, because the pH is below 7.
Check 21
What does a dilute solution contain?
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Only a small amount of dissolved substance in the volume.
Check 22
Does adding water make a strong acid into a weak acid?
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No. It only makes the solution more dilute.
Check 23
What do the units mol/dm³ measure?
Show answer
Concentration, the moles dissolved in 1 dm³.
Check 24
Why does a strong acid conduct electricity well?
Show answer
It has many ions free to carry the current.
Check 25
How many times more hydrogen ions are there at pH 1 than at pH 2?
Show answer
10 times.
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