OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 22

13 marks · Medium difficulty · Calculations

Calculate the mass of magnesium needed to react with phosphoric acid, determine the volume of phosphine gas produced using the ideal gas equation, and write related chemical equations.

Practise this question

Question

A multi-part chemistry exam question about compounds of magnesium and phosphorus. Part (a) involves the reaction between magnesium and phosphoric acid, requiring explanations of electron transfer, stoichiometric calculations for mass, methods for obtaining a solid product, and writing an equation using a suitable magnesium compound. Part (b) concerns the thermal decomposition of phosphorous acid to form phosphine gas, requiring ideal gas equation calculations for volume and constructing a combustion equation for phosphine.
Question text

22 This question is about compounds of magnesium and phosphorus.

(a) A student plans to prepare magnesium phosphate using the redox reaction of magnesium

with phosphoric acid, H3PO4.

3Mg(s) + 2H3PO4(aq) → Mg3(PO4)2(s) + 3H2(g)

(i) In terms of the number of electrons transferred, explain whether magnesium is being

oxidised or reduced.

… [1]

(ii) The student plans to add magnesium to 50.0 cm3 of 1.24 mol dm−3 H PO .

Calculate the mass of magnesium that the student should add to react exactly with the

phosphoric acid.

Give your answer to three significant figures.

mass of Mg = … g [3]

(iii) How could the student obtain a sample of magnesium phosphate after reacting

magnesium with phosphoric acid?

… [2]

(iv) Magnesium phosphate can also be prepared by reacting phosphoric acid with a

compound of magnesium.

Choose a suitable magnesium compound for this preparation and write the equation for

the reaction.

Formula of compound …

Equation … [2]

(b) Phosphine, PH3, is a gas formed by heating phosphorous acid, H3PO3, in the absence of air.

4H3PO3(s) → PH3(g) + 3H3PO4(s)

(i) 3.20 × 10−2 mol of H PO is completely decomposed by this reaction.

Calculate the volume of phosphine gas formed, in cm3, at 100 kPa pressure and 200 °C.

volume of PH = … cm3 [4]

(ii) When exposed to air, phosphine spontaneously ignites, forming P4O10 and water.

Construct an equation for this reaction.

… [1]

Mark scheme

Show the mark scheme The mark scheme provides detailed guidance and answers for all parts of question 22. It lists accepted answers for electron transfer, step-by-step mole and mass calculations for magnesium, separation and drying techniques for magnesium phosphate, acceptable alternative magnesium compounds and equations, ideal gas equation calculations including unit conversions for phosphine volume, and balanced equations for phosphine combustion.

Question Answer Marks Guidance

22 (a) (i) Oxidised 1 ALLOW Mg loses 6 electrons: 3 Mg in equation

AND ALLOW Mg → Mg2+ + 2e–

(Mg) transfers/loses/donates 2 electrons

2 essential IGNORE oxidation numbers (even if wrong)

(a) (ii) FIRST CHECK ANSWER ON THE ANSWER LINE 3

IF answer = 2.26 (3 SF) award 3 marks

----------------------------------------------------------------------------

1.24 50.0 At least 3SF needed throughout BUT

n(H3PO4) = 1000 = 0.062(0) (mol) ALLOW no trailing zeroes (e.g. 0.062 for 0.0620)

n(Mg) = 2 0.062(0) = 0.093(0) (mol) ALLOW ECF from n(H3PO4)

mass of Mg = 0.0930 24.3 = 2.26 (g) ALLOW ECF from n(Mg)

3 SF required ---------------------------------------------

COMMON ERRORS for 2 marks

3:2 ratio omitted n(Mg) = 0.062(0) 1.51 (g)

Inverted 2:3 ratio n(Mg) = 0.0413 1.00 (g)

(a) (iii) Separation of solid 2 ALLOW

Filter to obtain solid/precipitate Removal of water

Requires realisation that solid is filtered off. Evaporate/ distil water/solution/liquid

Solid may be stated within in ‘removal of water’ IGNORE ‘distil’ if product OR H2 is distilled

Removal of water Collection of remaining solid

Dry (solid) Requires realisation that solid remains

OR Evaporate (water/solution/liquid) IGNORE ‘Leave to crystallise’ (already solid)

(a) (iv) Formula 2 In equation:

MgO OR Mg(OH)2 OR MgCO3 OR soluble Mg salt NO ECF from incorrect formula

Equation ALLOW multiples

3MgO + 2H3PO4 Mg3(PO4)2 + 3H2O IGNORE state symbols (even if incorrect)

OR

3Mg(OH)2 + 2H3PO4 Mg3(PO4)2 + 6H2O Soluble Mg salts include

OR MgCl2, MgSO4, Mg(NO3)2, MgBr2, MgI2

3MgCO3 + 2H3PO4 Mg3(PO4)2 + 3CO2 + 3H2O If unsure, check with TL

e.g. 3MgCl2 + 2H3PO4 Mg3(PO4)2 + 6HCl

(b) (i) FIRST CHECK ANSWER ON THE ANSWER LINE 4 If there is an alternative answer, check to see if

IF answer = 315 (cm3) award 4 marks 9 there is any ECF credit possible

-----------------------------------------------------------------------------

Amount of PH3

3.20 10–2 ALLOW ECF throughout

n(PH ) = OR 8(.00) 10–3 (mol)

34 ------------------------------------------------------------------

Common Errors (3 marks)

Unit conversions

3 Use of n(H PO ) = 3.20 10–2 (Very common)

p conversion Pa = 100 × 10 (Pa) 3 4

3.2(0) 10–2 × 8.314 × 473

AND 6

V = 3 10

T conversion K = 473 (K) 100 10

= 1258.40704 cm3 (1260 to 3 SF)

Evidence of use of rearranged gas equation

nRT No temperature conversion from ºC to K

OR V = p –3

8(.00) 10 × 8.314 × 200 6

–3 V = 100 × 103 10

8(.00) 10 × 8.314 × 473

OR V = 100 × 103 = 133 cm3

OR V = 3.15 10–4

Calculator: = 3.1460176 10–4 No p conversion from kPa to Pa

8(.00) 10–3 × 8.314 × 473

V = 106

V conversion of m3 cm3 100

–4 6 3 = 315000 cm3

V = 3.15 10 10 = 315 cm

33 No volume conversion from m3 to cm3

Calculator from unrounded cm : 314.60176 cm

V = 3.15 10–4

Requires 3 OR MORE SF, correctly rounded

ALLOW use of R = 8.31 314.4504 314 to 3SF IGNORE use of 24/24000 for molar volume e.g.

3.2(0) 10–3 24000 = 768 scores zero

8(.00) 10–3 24000 = 292 scores 1st mark only

(b) (ii) 4PH3 + 8O2 P4O10 + 6H2O 1 ALLOW multiples

Total 13

How to answer it

Compounds of Magnesium and Phosphorus

What this question tests

This question assesses core AS Chemistry competencies: defining oxidation in terms of electron transfer, executing multi-step stoichiometry calculations with solution concentrations, describing practical laboratory separation techniques, writing balanced equations for acid-base neutralisations, applying the Ideal Gas Equation (pVT = nRT) with strict unit conversions, and constructing combustion equations.

Question 2 (a) (i)

Electron Transfer in Redox Reactions

✅ Correct Answer

Magnesium is oxidised AND it loses/transfers/donates 2 electrons (per atom, or Mg → Mg²⁺ + 2e⁻ ).

💡 Key Knowledge

Remember the acronym OIL RIG (Oxidation Is Loss of electrons). In its elemental state ( Mg ), magnesium has an oxidation number of 0, which increases to +2 in Mg₃(PO₄)₂ .

❌ Common Errors

Stating that magnesium is reduced, or failing to mention the specific number of electrons (the mark scheme strictly insists on the number '2'). Note: mentioning oxidation numbers is ignored if the electron transfer is stated incorrectly.

🎯 Mark: 1 mark
Question 2 (a) (ii)

Stoichiometry Calculation: Mass of Magnesium

📐 Step-by-Step Calculation

  1. Find moles of H₃PO₄:
    n(H₃PO₄) = (1.24 × 50.0) / 1000 = 0.0620 mol
  2. Use the stoichiometric ratio (3:2):
    From 3Mg + 2H₃PO₄ , ratio is 3 mol Mg : 2 mol H₃PO₄.
    n(Mg) = (3 / 2) × 0.0620 = 0.0930 mol
  3. Calculate mass of Mg:
    mass = n × Mr = 0.0930 × 24.3 = 2.2569 g
  4. Apply Significant Figures:
    Round to 3 SF: 2.26 g

🧠 Exam Technique & Traps

  • Answer Line Check: Examiners check the answer line first. If 2.26 is written, full marks are awarded instantly.
  • The 3:2 Ratio Trap: A very common error is using an incorrect 1:1 mole ratio, giving 1.51 g instead of 2.26 g (costs 1 mark).
  • Significant Figures: The question explicitly requests three significant figures. Do not round prematurely in intermediate steps!
🎯 Marks: 3 marks
Question 2 (a) (iii)

Practical Separation of Magnesium Phosphate

✅ Correct Answer

1. Separation of solid: Filter to obtain the solid precipitate.
2. Removal of water: Dry the solid (or evaporate the water/solution).

❌ Common Errors

Stating "leave to crystallise". Because magnesium phosphate is produced as an insoluble solid precipitate directly in the mixture, crystallisation is incorrect—you must physically filter to isolate it.

🎯 Marks: 2 marks
Question 2 (a) (iv)

Alternative Preparation of Magnesium Phosphate

✅ Correct Answers

Formula of compound: MgO , Mg(OH)₂ , MgCO₃ , or any soluble Mg salt (e.g., MgCl₂ ).

Example Equation (using MgO):
3MgO(s) + 2H₃PO₄(aq) → Mg₃(PO₄)₂(s) + 3H₂O(l)

💡 Key Knowledge

Acids react with metal oxides, hydroxides, and carbonates to form salts. Ensure your chosen formula matches your balanced stoichiometric equation.

🎯 Marks: 2 marks
Question 2 (b) (i)

Ideal Gas Equation Calculation

📐 Step-by-Step Calculation

  1. Moles of PH₃:
    From equation, 4 mol H₃PO₃ yields 1 mol PH₃.
    n(PH₃) = (3.20 × 10⁻²) / 4 = 8.00 × 10⁻³ mol
  2. Convert Units:
    Pressure ( p ) = 100 kPa = 100 × 10³ Pa
    Temperature ( T ) = 200 + 273 = 473 K
  3. Rearrange & Solve pV = nRT:
    V = (nRT) / p
    V = (8.00 × 10⁻³ × 8.314 × 473) / (100 × 10³)
    V = 3.146 × 10⁻⁴ m³
  4. Convert m³ to cm³:
    Multiply by 10⁶ : 3.146 × 10⁻⁴ × 10⁶ = 314.6 cm³
    Round to 3 SF: 315 cm³ (Using R = 8.3 gives 314 cm³ ).

❌ Common Calculation Traps

  • Using wrong moles: Using 3.20 × 10⁻² directly without dividing by 4 (omitting the 4:1 stoichiometric ratio).
  • Temperature slip: Forgetting to convert Celsius to Kelvin ( +273 ), using 200 K instead of 473 K .
  • Pressure conversion omission: Forgetting to multiply kPa by 10³ to get Pascals.
  • Volume unit trap: Forgetting to scale up from m³ to cm³ (missing the × 10⁶ factor).
🎯 Marks: 4 marks
Question 2 (b) (ii)

Combustion Equation for Phosphine

✅ Correct Answer

4PH₃ + 8O₂ → P₄O₁₀ + 6H₂O

🧠 Exam Technique

Balance elements systematically: start with phosphorus, then hydrogen, and leave oxygen ( O₂ ) until last to balance fractional or whole integers easily. Multiples are accepted.

🎯 Mark: 1 mark

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.1 Atoms and reactions · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.