Edexcel A-Level Chemistry Paper 1, November 2020: Question 5

12 marks · Medium difficulty · Calculations

Investigate the thermal decomposition and water of crystallization of hydrated magnesium nitrate through observations, explanations of thermal stability trends, dot-and-cross diagrams, and stoichiometric calculations.

Practise this question

Question

A multi-part chemistry question about hydrated magnesium nitrate, Mg(NO3)2.xH2O. Part (a) asks to state two observations when it is heated, and to explain the trend in thermal stability of Group 2 nitrates. Part (b) gives experimental data where 0.765 g of the sample is dissolved and reacted with sodium hydroxide to produce 0.174 g of magnesium hydroxide precipitate, followed by drawing dot-and-cross diagrams for the ions and calculating the value of x.
Question text

5 This question is about the chemistry of hydrated magnesium nitrate, Mg(NO3)2.xH2O.

(a) Group 2 nitrates decompose when heated.

(i) State two observations you would see when hydrated magnesium nitrate is heated.

(2)

(ii) Explain the trend in thermal stability of Group 2 nitrates.

(3)

(b) In an experiment, a sample of hydrated magnesium nitrate, Mg(NO3)2.xH2O,

with a mass of 0.765g, was dissolved in water and reacted with an excess of

sodium hydroxide solution, NaOH(aq).

The precipitate of magnesium hydroxide, Mg(OH)2, produced was removed and

dried. The mass of the dried sample was 0.174g.

(i) Draw dot-and-cross diagrams for the ions in magnesium hydroxide.

Show the outer electrons only.

(2)

(ii) Use the experimental data to calculate the value for x in the formula

Mg(NO3)2.xH2O.

You must show all your working.

(5)

*P62668A01424*

(Total for Question 5 = 12 marks)

Mark scheme

Show the mark scheme The mark scheme shows acceptable answers for each part. For 5(a)(i), it lists solid dissolves, condensation, brown gas, and white solid. For 5(a)(ii), it references ionic radius, polarising ability, and N-O bond weakening. For 5(b)(i), it provides the dot-and-cross diagram showing a Mg 2+ ion with no outer electrons and two hydroxide ions. For 5(b)(ii), it outlines the step-by-step calculation showing moles of Mg(OH)2, mass of Mg(NO3)2, mass of water, moles of water, and the final ratio giving x = 6.

How to answer it

Chemistry of Hydrated Magnesium Nitrate

What this question tests

This question evaluates your understanding of Group 2 chemistry, specifically thermal decomposition observations and the periodic trend in thermal stability driven by ionic radii and polarising ability. It also tests practical chemistry skills, ionic bonding dot-and-cross diagrams, and stoichiometry/water of crystallisation calculations.

Part (a)(i) — Thermal Decomposition Observations

Observations when heating hydrated magnesium nitrate

✅ Correct Answers (Any 2)

  • Solid dissolves / melts
  • Condensation on the sides of the test tube
  • Brown gas / brown fumes / brown NO₂ produced
  • White solid / powder forms

❌ Common Errors

  • Saying "magnesium dissolved" or "solid disappears" (lost mark — it melts in its water of crystallisation).
  • Writing "magnesium oxide forms" (this is an interpretation/formula, not a visual observation).
  • Using imprecise terms like "precipitate" for a solid forming from solution.
Marks available: 2 marks (1 mark per valid observation).
Part (a)(ii) — Trend in Thermal Stability

Explaining Group 2 Nitrate Thermal Stability

💡 Key Knowledge

  • Trend: Thermal stability increases down Group 2.
  • Ionic radius: Ionic radius of the metal cation increases down the group.
  • Polarising ability: Charge density decreases, so the cation's ability to polarise / distort the electron cloud of the nitrate anion decreases.
  • Bond weakening: The weakening of N-O bonds within the nitrate ion is less pronounced down the group.

🧠 Exam Technique

To score all 3 marks, structure your answer logically using cause-and-effect: Down the group → Cation radius increases → Polarising power decreases → Nitrate electron cloud distorted less → N-O bonds less weakened. Avoid vague phrases like "atomic radius increases" (must refer to ions).

Marks available: 3 marks.
Part (b)(i) — Ionic Bonding Diagrams

Dot-and-Cross Diagrams for Magnesium Hydroxide Ions

✅ Correct Answers

  • Magnesium ion (Mg²⁺): Show formula [Mg]²⁺ with outer shell shown empty (no electrons), or showing a full preceding shell.
  • Hydroxide ion (OH⁻): Show formula [O—H]⁻ with a single covalent bond (one dot, one cross) and lone pairs on oxygen, enclosed in square brackets with a negative charge outside. Two hydroxide ions are required or implied by stoichiometry.

❌ Common Errors

  • Forgetting to include square brackets and charges around the ions.
  • Showing electrons in the outer shell of the Mg²⁺ ion.
  • Failing to show the shared pair of electrons in the O-H covalent bond clearly.
Marks available: 2 marks (1 for magnesium ion, 1 for hydroxide ion).
Part (b)(ii) — Stoichiometry Calculation

Calculating the Value of x in Mg(NO₃)₂ · xH₂O

📐 Step-by-Step Calculation

  1. Find moles of Mg(OH)₂ precipitate:
    Molar mass of Mg(OH)₂ = 24.3 + (16.0 + 1.0) × 2 = 58.3 g mol⁻¹
    Moles = 0.174 g / 58.3 g mol⁻¹ = 2.985 × 10⁻³ mol
  2. Relate to anhydrous Mg(NO₃)₂:
    Since 1 mole of Mg(OH)₂ comes from 1 mole of Mg(NO₃)₂, moles of Mg(NO₃)₂ = 2.985 × 10⁻³ mol
    Molar mass of Mg(NO₃)₂ = 24.3 + (14.0 + (16.0 × 3)) × 2 = 148.3 g mol⁻¹
    Mass of Mg(NO₃)₂ = 2.985 × 10⁻³ mol × 148.3 g mol⁻¹ = 0.4426 g
  3. Calculate mass and moles of water of crystallisation (H₂O):
    Mass of H₂O = Total mass of hydrated sample − Mass of Mg(NO₃)₂
    Mass of H₂O = 0.765 g − 0.4426 g = 0.3224 g
    Moles of H₂O = 0.3224 g / 18.0 g mol⁻¹ = 0.01791 mol
  4. Determine the ratio (x):
    Ratio of Mg(NO₃)₂ : H₂O = 2.985 × 10⁻³ : 0.01791 = 1 : 6.00
    Therefore, x = 6

❌ Common Calculation Traps

  • Rounding intermediate numbers too early, which skews the final integer value for x.
  • Using atomic mass values with inconsistent significant figures (use Periodic Table values: Mg = 24.3, N = 14.0, O = 16.0, H = 1.0).

🧠 Exam Technique Tip

Always state your final whole number clearly for x at the end of the calculation. If your calculated ratio comes out as 5.98 or 6.02, round sensibly to the nearest whole integer (6).

Marks available: 5 marks.

Topics

Inorganic Chemistry · Physical Chemistry · Organic Chemistry · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 5: Formulae, Equations and Amounts of Substance · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.