AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2023: Question 5

12 marks · Standard Demand difficulty · Short Answer

This multi-part question covers properties of acids, titration calculations, ion identification, and electronic structure.

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Question

A multi-part chemistry exam question (05.1 to 05.7) regarding acids, alkalis, titration, and atomic structure. It includes a calculation involving a chemical equation for the reaction between sodium carbonate and nitric acid, and a question about hydrogen ion concentration in relation to pH.
Question text

05 This question is about acids and alkalis.

05.1 Ethanoic acid is a weak acid.

What is meant by ‘weak acid’?

Answer in terms of ionisation.

[1 mark]

05.2 The concentration of an acid can be measured in mol/dm3.

Which combination of changes increases the concentration of an acid?

[1 mark]

Tick ( ) one box.

The mass of acid dissolved is halved and the volume of the

solution is halved.

The mass of acid dissolved is halved and the volume of the

solution is doubled.

The mass of acid dissolved is doubled and the volume of the solution

is halved.

The mass of acid dissolved is doubled and the volume of the solution

is doubled.

05.3 The concentration of an acid can be determined by titration.

An indicator is added to an alkali in a flask.

Name an indicator that can be used in this titration.

Give the colour change of the indicator when acid from a burette is added to the alkali

in the flask.

[2 marks]

Name of indicator

Colour change from 15 to

05.4 Sodium carbonate dissolves in water to produce an alkaline solution.

Give the formula of the ion that makes a solution alkaline.

*14* [1 mark]

05.5 A student does a titration using sodium carbonate solution and nitric acid.

The equation for the reaction is:

Na2CO3 + 2 HNO3 → 2 NaNO3 + CO2 + H2O

25.0 cm3 of 0.124 mol/dm3 sodium carbonate solution is neutralised by

23.6 cm3 of nitric acid.

Calculate the concentration of the nitric acid.

Give your answer to 3 significant figures.

You should calculate:

• the number of moles of sodium carbonate in 25.0 cm3 of the solution

• the number of moles of nitric acid in 23.6 cm3 of the nitric acid

• the concentration of the nitric acid in mol/dm3.

[5 marks]

16 3

Concentration (3 significant figures) = mol/dm

When hydrochloric acid dissolves in water, hydrogen ions (H+) and chloride ions (Cl−)

are produced.

05.6 A solution of hydrochloric acid with pH 4.5 has a concentration of H+ ions of

*15* 3.16 × 10−5 mol/dm3.

What is the concentration of H+ ions in a solution of hydrochloric acid with pH 2.5?

[1 mark]

Concentration of H+ ions = mol/dm3

05.7 Which element has atoms that have the same electronic structure as the chloride ion?

Use the periodic table.

[1 mark]

Mark scheme

Show the mark scheme The mark scheme provides the correct answers and marking criteria for parts 05.1 through 05.7. It details the steps for the titration calculation, including the mole ratio, and specifies acceptable indicators and colour changes.

AO /

Question Answers Extra information Mark

Spec. Ref.

05.1 (the acid is only) partially ionised 1 AO1

(in aqueous solution) 4.4.2.6

AO /

Spec. Ref.

05.2 the mass of acid dissolved is 1 AO2

doubled and the volume of the 4.3.2.5

solution is halved

AO /

Spec. Ref.

05.3 MP2 is dependent on the award AO1

of MP1 4.4.2.5

RPA2

methyl orange 1

from yellow to red / orange / 1

pink

OR

phenolphthalein (1)

from pink to colourless (1)

OR

litmus (1)

from blue to red (1)

if no other marks awarded,

allow 1 mark for universal

indicator turns from–HEMISTRYpurple / blue– –

to green / yellow / orange / red

AO /

Spec. Ref.

– HEMISTRY – –

− 1 AO1

05.4 OH

16 4.4.2.4

AO /

Spec. Ref.

05.5 25.0 AO2

(moles Na2CO3 = × 0.124)

1000 4.3.4

1 4.4.2.5

= 0.0031(0)

RPA2

(moles HNO3 = 2 × 0.0031(0)) allow correct use of an 1

= 0.0062(0) incorrectly determined number

of moles of Na2CO3

0.0062(0)

(concentration =) ×1000 allow correct use of an 1

23.6

incorrectly determined number

of moles of HNO3

= 0.262711864 1

= 0.263 (mol/dm3) allow an answer correctly 1

rounded to 3 significant figures

from an incorrect calculation

which uses all the data in the

question

alternative approach:

moles HNO3

(ratio =) allow inverted expression

moles Na2CO3

2 23.6 × concentration allow 1 mark for the expression

= (2) with an incorrect mole ratio

1 25.0 × 0.124

(concentration =)

2 × 25.0 × 0.124 allow correct use of the

(1) expression with an incorrect

23.6

mole ratio

= 0.262711864 (1)

= 0.263 (mol/dm3) (1) allow an answer correctly

rounded to 3 significant figures

from an incorrect calculation 17

which uses all the data in the

question– HEMISTRY – –

18 AO /

Spec. Ref.

05.6 3.16 × 10−3 (mol/dm3) 1 AO2

4.4.2.6

AO /

Spec. Ref.

05.7 argon / Ar 1 AO2

4.1.2.4

4.2.1.2

– HEMISTRY – –

Total Question 5 12

Question 6

How to answer it

Acids, Alkalis, and Titration Calculations

What this question tests

This question assesses your understanding of acid strength (ionisation), concentration variables, titration practical skills (indicators and multi-step calculations), the pH scale (logarithmic H⁺ concentration), and electronic structures of ions.

Part 5.1 & 5.2

Acid Strength and Concentration

💡 Key Knowledge: Weak Acids

A weak acid is only partially ionised in aqueous solution. This means only a small fraction of the acid molecules break into ions (H⁺).

🧠 Exam Technique: Concentration

Concentration = Mass ÷ Volume. To increase concentration:

  • Increase the mass of solute.
  • Decrease the volume of solvent.

✅ Correct Answers

  • 5.1: (The acid is only) partially ionised.
  • 5.2: The mass of acid dissolved is doubled and the volume of the solution is halved.
Part 5.3 & 5.4

Titration Indicators and Ions

🧠 Indicator Choice

In a titration, you must use a single indicator that gives a sharp colour change. Universal Indicator is never used because its colour change is too gradual.

❌ Common Error: Colour Order

The question says acid is added to alkali. This means the flask starts alkaline and ends acidic. Your colour change must reflect this order!

✅ Correct Answers

5.3: (Choose one pair)

  • Methyl orange: Yellow to Red (or orange/pink).
  • Phenolphthalein: Pink to Colourless.

5.4: OH⁻ (The hydroxide ion makes a solution alkaline).

Part 5.5

Titration Calculation (5 Marks)

Equation: Na₂CO₃ + 2 HNO₃ → 2 NaNO₃ + CO₂ + H₂O

📐 Step-by-Step Calculation

  1. Find moles of Sodium Carbonate:
    Moles = (Volume ÷ 1000) × Concentration
    (25.0 ÷ 1000) × 0.124 = 0.0031 mol
  2. Find moles of Nitric Acid (using the ratio):
    The equation shows a 1:2 ratio (1 Na₂CO₃ reacts with 2 HNO₃).
    0.0031 × 2 = 0.0062 mol
  3. Calculate Concentration of Nitric Acid:
    Conc = (Moles × 1000) ÷ Volume
    (0.0062 × 1000) ÷ 23.6 = 0.262711...
  4. Round to 3 Significant Figures:
    Answer = 0.263 mol/dm³

❌ Calculation Traps

  • The 1000 Rule: Always divide cm³ by 1000 to get dm³ before calculating moles.
  • The Ratio: Many students forget to multiply by 2. Look at the big numbers in the equation!
  • Sig Figs: 0.26 is only 2 sig figs. You must provide 0.263 for the final mark.
Part 5.6 & 5.7

pH Scale and Atomic Structure

💡 The pH Factor of 10

Every 1 unit change in pH represents a 10x change in H⁺ ion concentration.

  • pH 4.5 to 3.5 = 10x more H⁺
  • pH 4.5 to 2.5 = 100x more H⁺ (10 × 10)

🧠 Electronic Structure of Ions

A Chlorine atom (Cl) is in Group 7 (2,8,7). A chloride ion (Cl⁻) has gained one electron to get a full outer shell (2,8,8). Look for the Noble Gas in Period 3!

✅ Correct Answers

5.6: 3.16 × 10⁻³ mol/dm³
(Calculated as: 3.16 × 10⁻⁵ × 100)

5.7: Argon / Ar

Topics

Chemistry · Required Practicals · Required Practicals

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.