Edexcel International A Level Chemistry: Practical Skills in Chemistry I (YCH11) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

Every question below targets Unit 3: Practical Skills in Chemistry I from the YCH11 specification (Pearson Edexcel, International Advanced Subsidiary/Advanced Level in Chemistry, Issue 1, September 2017). It is an International AS (IAS) unit covering the skills from Units 1 and 2, including Core Practicals 1 to 8. The methods are taught in the study guide and summarised in the revision notes. Plan with the course hub, checklist and free diagnostics.

Data for this set: H = 1.0, C = 12.0, O = 16.0, Na = 23.0, S = 32.1, Cl = 35.5, Ca = 40.1, Cu = 63.5. Gas constant R = 8.31 J mol⁻¹ K⁻¹. Specific heat capacity of solutions = 4.18 J g⁻¹ °C⁻¹.

Questions

1. Uncertainties per reading: balance ±0.005 g; burette ±0.05 cm³.

(a) 0.240 g of a solid is weighed by difference. Calculate the percentage uncertainty in the mass. [1] (b) Suggest how to reduce this uncertainty with the same balance. [1] (c) Burette readings are 0.85 cm³ (start) and 19.15 cm³ (end). Calculate the titre and its percentage uncertainty. [2]

2. Core Practical 1. 0.350 g of calcium carbonate reacts with excess dilute hydrochloric acid, giving 79.5 cm³ of carbon dioxide, collected over water at 20 °C and 100 kPa.

(a) Give a balanced equation, including state symbols, for this reaction. [1] (b) Find the molar volume of the gas in dm³ mol⁻¹. [2] (c) Use pV = nRT to calculate the expected molar volume at 20 °C and 100 kPa. [2] (d) Suggest a reason, other than a leak, for the low result. [1]

3. Core Practical 2. You want ΔH for CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s).

• 4.00 g of anhydrous CuSO₄ dissolves in 50.0 cm³ of water: rise of 7.8 °C.
• 6.24 g of CuSO₄·5H₂O dissolves in 50.0 cm³ of water: fall of 1.3 °C.

(a) Calculate ΔH for dissolving each solid, in kJ mol⁻¹. [4] (b) Use a Hess cycle to calculate ΔH for the hydration. [1] (c) Explain why the hydration cannot be measured directly. [1]

4. Core Practical 3. 25.0 cm³ of hydrochloric acid is titrated with 0.0950 mol dm⁻³ sodium hydroxide using methyl orange. Titres: rough 26.40, then 25.85, 26.10 and 25.95 cm³.

(a) Give the colour change seen when the end point is reached. [1] (b) Calculate the mean titre to use. [1] (c) Calculate the concentration of the acid in mol dm⁻³. [2] (d) Convert your answer to g dm⁻³. [1]

5. Core Practical 4.

(a) Describe how to make 250 cm³ of a standard solution from a weighed sample of a solid acid. [4] (b) Explain why the burette is rinsed with the sodium hydroxide solution, not with water. [1]

6. Core Practical 5. Three isomers of C₄H₉Br are tested with silver nitrate in ethanol at 40 °C. Times for a precipitate: 1-bromobutane 520 s; 2-bromobutane 190 s; 2-bromo-2-methylpropane 15 s.

(a) State the colour of the precipitate. [1] (b) Calculate the relative rate, 1/time, for 2-bromo-2-methylpropane. [1] (c) State the trend shown, using the terms primary, secondary and tertiary. [1] (d) Explain why ethanol is used. [1] (e) State one variable, other than temperature, that must be controlled. [1]

7. Core Practical 6. 10.0 cm³ of 2-methylpropan-2-ol (density 0.786 g cm⁻³) is shaken with excess concentrated hydrochloric acid, giving 6.24 g of pure 2-chloro-2-methylpropane (density 0.84 g cm⁻³, boiling temperature 51 °C).

(a) Is the product in the top or bottom layer of the funnel? Give a reason. [1] (b) The crude product is shaken with sodium hydrogencarbonate solution. Give the purpose of this step and explain why the funnel is vented often. [2] (c) State how you know the product is dry. [1] (d) Calculate the percentage yield. [3] (e) State a suitable collection range for the product. [1]

8. Core Practical 7. A student oxidises propan-1-ol using potassium dichromate(VI) in dilute sulfuric acid.

(a) Explain why propanal is distilled off as it forms, into a cooled receiver. [2] (b) Give a test, with the result, that shows the distillate contains an aldehyde. [1] (c) Give a test, with the result, for propanoic acid. [1] (d) State the colour change of the oxidising agent. [1] (e) State one hazard in this practical and a matching precaution. [1]

9. Core Practical 8. Three unknowns are tested.

• Solid W: lilac flame; white precipitate with dilute hydrochloric acid then barium chloride.
• Solid Y: fizzes with dilute acid, giving a gas that turns limewater milky; warmed with sodium hydroxide, gives a gas that turns damp red litmus blue.
• Liquid Z, C₃H₈O: steamy fumes with phosphorus(V) chloride; turns warm acidified dichromate(VI) orange to green; the product, distilled off as it forms, does not affect Benedict’s solution.

(a) Identify W. Give the ionic equation for the barium chloride test. [2] (b) Identify Y. Give the ionic equation for the reaction with sodium hydroxide. [2] (c) Identify Z, explaining how each observation supports your answer. [3] (d) Explain why the barium chloride is added after acid. [1]

10. A 250 cm³ solution is made from 3.58 g of Na₂CO₃·xH₂O. Each 25.0 cm³ portion needs, on average, 25.00 cm³ of 0.100 mol dm⁻³ hydrochloric acid (methyl orange). Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O.

(a) Calculate the value of x. [5] (b) Explain why methyl orange is used, not phenolphthalein. [1] (c) Suggest how to reduce the percentage uncertainty in the titre. [1]

Answers

1. (a) 2 × 0.005 = 0.010 g; 0.010 ÷ 0.240 × 100 = 4.2% [1]. (b) Weigh a larger mass of solid [1]. (c) Titre = 19.15 − 0.85 = 18.30 cm³ [1]; 0.10 ÷ 18.30 × 100 = 0.55% [1]. Examiner insight: Show the doubling of the reading uncertainty; a single-reading value of 0.05 gives half the correct percentage.

2. (a) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) [1]. (b) n = 0.350 ÷ 100.1 = 3.497 × 10⁻³ mol [1]; 79.5 ÷ 3.497 × 10⁻³ = 22 700 cm³ mol⁻¹ = 22.7 dm³ mol⁻¹ [1]. (c) V = RT ÷ p = 8.31 × 293 ÷ 100 000 [1] = 0.0243 m³ = 24.3 dm³ mol⁻¹ [1]. (d) Some carbon dioxide dissolves in the water it is collected over [1]. Examiner insight: Use kelvin and pascals in pV = nRT, then convert m³ to dm³; mixed units cannot give the right value.

3. (a) Anhydrous: energy = 50.0 × 4.18 × 7.8 = 1630 J [1]; n = 4.00 ÷ 159.6 = 0.02506 mol; ΔH₁ = −1.630 ÷ 0.02506 = −65.0 kJ mol⁻¹ [1]. Hydrated: energy = 50.0 × 4.18 × 1.3 = 272 J [1]; n = 6.24 ÷ 249.6 = 0.0250 mol; ΔH₂ = +10.9 kJ mol⁻¹ [1]. (b) ΔH = ΔH₁ − ΔH₂ = −65.0 − 10.9 = −75.9 kJ mol⁻¹ [1]. (c) Adding exactly five moles of water without dissolving some solid is impossible [1]. Examiner insight: A rise gives a negative ΔH and a fall a positive one; write the sign on every value.

4. (a) Red to orange [1]. (b) 25.85 and 25.95 are concordant; mean = 25.90 cm³ [1]. (c) n(NaOH) = 0.0950 × 0.02590 = 2.461 × 10⁻³ mol = n(HCl) [1]; c = 2.461 × 10⁻³ ÷ 0.0250 = 0.0984 mol dm⁻³ [1]. (d) 0.0984 × 36.5 = 3.59 g dm⁻³ [1]. Examiner insight: Show which titres you chose; quote the mean to two decimal places, like the readings.

5. (a) Weigh by difference, then dissolve the solid in a beaker of distilled water [1]. Move the solution and all beaker and rod washings into a 250 cm³ volumetric flask [1]. Make up until the meniscus bottom sits on the mark at eye level [1]. Stopper the flask and invert it several times [1]. (b) Water would dilute the alkali and change its concentration [1]. Examiner insight: Give the detail that makes each step work: “rinse the beaker into the flask” is stronger than “transfer it”.

6. (a) Cream [1]. (b) 1 ÷ 15 = 0.067 s⁻¹ [1]. (c) Rate increases from primary to secondary to tertiary [1]. (d) Ethanol dissolves both the halogenoalkane and the aqueous reagent [1]. (e) Concentration and volume of silver nitrate solution, or amount of each halogenoalkane [1]. Examiner insight: Name the controlled variable precisely; “the amounts” alone is weak.

7. (a) Top: 0.84 g cm⁻³ is below the density of the aqueous layer [1]. (b) Neutralises leftover acid [1]; it releases carbon dioxide, so pressure builds up [1]. (c) The liquid turns from cloudy to clear [1]. (d) Mass of alcohol = 10.0 × 0.786 = 7.86 g; n = 7.86 ÷ 74.0 = 0.1062 mol [1]. Theoretical mass = 0.1062 × 92.5 = 9.83 g [1]. Yield = 6.24 ÷ 9.83 × 100 = 63.5% [1]. (e) About 49 to 53 °C [1]. Examiner insight: Convert volume to mass with the density before finding moles.

8. (a) Removing propanal as it forms prevents further oxidation to propanoic acid [1]; propanal is volatile, so cooling reduces loss by evaporation [1]. (b) Warmed Benedict’s (or Fehling’s) solution: blue, then a red precipitate [1]. (c) Add sodium hydrogencarbonate: effervescence of a gas that turns limewater milky [1]. (d) Orange to green [1]. (e) Dichromate(VI) is toxic: gloves. Or the alcohol is flammable: no naked flame [1]. Examiner insight: Say what distilling prevents: further oxidation.

9. (a) Potassium sulfate, K₂SO₄ [1]; Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) [1]. (b) Ammonium carbonate, (NH₄)₂CO₃ (ammonium hydrogencarbonate also fits) [1]; NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l) [1]. (c) Steamy fumes: an –OH group, so an alcohol [1]. Orange to green: primary or secondary [1]. No aldehyde, so the product is a ketone and Z is secondary: propan-2-ol [1]. (d) Acid removes carbonate (or similar) ions that would also give a white precipitate [1]. Examiner insight: Ionic equations need state symbols and no spectator ions; including K⁺ or Cl⁻ weakens the answer.

10. (a) n(HCl) = 0.100 × 0.02500 = 2.50 × 10⁻³ mol [1]. n(Na₂CO₃) in 25.0 cm³ = 1.25 × 10⁻³ mol [1]. In 250 cm³: 1.25 × 10⁻² mol [1]. M = 3.58 ÷ 0.0125 = 286 g mol⁻¹ [1]. x = (286 − 106) ÷ 18.0 = 10 [1]. (b) Phenolphthalein changes colour at the hydrogencarbonate stage, after only half the acid needed [1]. (c) Use more dilute acid, so the titre is larger [1]. Examiner insight: Scale from 25.0 cm³ to 250 cm³ before finding M; missing the factor of 10 is an easy slip.

Where marks are usually lost

• Averaging every titre, including the rough one.
• Halving the percentage uncertainty by forgetting that a titre uses two readings.
• Losing the sign of ΔH, or giving it in J mol⁻¹ when kJ mol⁻¹ is asked for.
• Writing “turns green” with no starting colour.
• Giving “wear goggles” when a specific hazard is named.
• Leaving out the scale-up factor for a 25.0 cm³ portion of a 250 cm³ solution.

Next steps

• Unit 3 revision notes
• Unit 3 study guide
• Course hub for Edexcel IAL Chemistry
• Unit-by-unit checklist to print
• Book a free trial class
• Free 10-minute diagnostics

Official syllabus

Source: the YCH11 specification, Issue 1 (September 2017), Pearson Education Limited, for the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry. Unit 3: Practical Skills in Chemistry I, with Core Practicals 1 to 8 from Units 1 and 2.