QUESTIONS
1. You are provided with the following:
Solution A containing an aqueous solution of hydrochloric acid, HCl.
Solution B containing 0.02 mole dm⁻³ solution of sodium trioxocarbonate(IV).
Solution C containing an aqueous solution of sodium hydroxide.
Methyl orange indicator
(a) Procedure:
i) Pipette 20 or 25cm³ of solution B into a conical flask. Add 2 drops of indicator. From the burette, run in solution A to an orange end point. Record your titre value.
ii) Repeat the procedure using fresh portions of 20 or 25cm³ of solution B two more times. Find the average titre value of the titrations. (16 marks)
iii) Pipette 20 or 25cm³ of solution C into a conical flask. Add 2 drops of indicator. Titrate against solution A from the burette. Record your titre value.
iv) Repeat the titration 2 or more times and determine the average titre value. (2 marks)
(b) From your results, calculate the:
i) number of moles of hydrochloric acid in 1dm³ of A. (2 marks)
ii) concentration of moles of A. (2 marks)
iii) number of moles of sodium hydroxide in 1dm³ of C. (2 marks)
iv) mass concentration of solution C. (2 marks)
2. You are provided with sample X1 — Aluminium tetraoxosulphate(VI) salt. Carry out the following tests and record your observations clearly, giving the appropriate inferences for careful observations clearly recorded and explanatory notes on unusual reactions.
a) Heat a small amount of the sample. (Observation/Inference expected)
b) Dissolve a small amount of the sample with a drop of concentrated HCl and heat in a non-luminous flame, using a clean spatula.
c) Add concentrated tetraoxosulphate(VI) acid to sample. Warm and test any gas evolved with damp litmus paper.
d) Prepare a solution of the sample in a test tube and using small portions in each case, carry out the following tests:
i) add aqueous ammonia solution drop wise and then in excess.
ii) add aqueous solution of sodium hydroxide drop wise and then in excess.
iii) add few drops of aqueous barium chloride.
iv) add aqueous sodium tetraoxophosphate(V) solution drop wise and excess; then add dilute ethanoic acid.
v) add aqueous sodium trioxocarbonate(IV) solution drop wise and in excess.
e) In each of the positive tests observed in section (b), write a balanced equation for the process involved.
ANSWERS
Question 1 — Volumetric Analysis
(a) (Practical procedure — no numerical answer; the student records their own titre values from the actual titration.)
Sample worked calculation (using illustrative titre values as a guide to method):
(b) i) Number of moles of HCl in 1dm³ of A
Assume average titre of A (against 25cm³ of B, 0.02M Na₂CO₃) = V₁ cm³ (student's own reading)
Reaction: 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂
moles Na₂CO₃ used = 0.02 × (25/1000) = 0.0005 mol
moles HCl reacted = 2 × 0.0005 = 0.001 mol (this is the amount in V₁ cm³ of A)
Moles of HCl in 1dm³ (1000cm³) of A = 0.001 × (1000/V₁) mol
(Substitute the student's actual average titre value V₁ to obtain the numerical answer.)
ii) Concentration of A = moles per dm³ calculated in (i) directly, expressed in mol/dm³
iii) Number of moles of NaOH in 1dm³ of C
Reaction: HCl + NaOH → NaCl + H₂O (1:1 mole ratio)
Assume average titre of A (against 25cm³ of C) = V₂ cm³
moles HCl used = concentration of A (from part i) × (V₂/1000)
moles NaOH = moles HCl (1:1 ratio) = same value
Moles of NaOH in 1dm³ of C = [moles HCl used × (1000/25)] mol
(Substitute actual titre V₂ and the concentration of A found in (i) to compute the numerical value.)
iv) Mass concentration of C
Mass concentration (g/dm³) = moles/dm³ (from iii) × molar mass of NaOH (40 g/mol)
Mass concentration = [moles/dm³] × 40 g/dm³
Question 2 — Qualitative Analysis: Sample X1 (Aluminium tetraoxosulphate(VI) — Al₂(SO₄)₃)
a) Heat a small amount of the sample:
Observation: The white solid decrepitates/swells slightly and then remains largely unchanged or turns to a white anhydrous powder (loses water of crystallisation, if hydrated) — no colour change, no gas with characteristic smell at moderate heat, though at very high heat SO₃ fumes may be released.
Inference: Consistent with a hydrated sulphate salt of a metal with no easily reducible/oxidisable cation (e.g., Al³⁺).
b) Dissolve with conc. HCl and heat in a non-luminous flame using a clean spatula (flame test):
Observation: No distinctive/characteristic flame colouration is observed (Al³⁺ gives no characteristic flame colour, as it is not a typical flame test cation).
Inference: Absence of flame colour is consistent with Al³⁺ (rules out Na⁺-yellow, K⁺-lilac, Ca²⁺-brick red, Cu²⁺-blue-green, etc.)
c) Add concentrated H₂SO₄, warm, test any gas evolved with damp litmus paper:
Observation: No characteristic acidic gas (e.g., no steamy white fumes of HCl, no brown fumes of NO₂, no smell of SO₂) is observed, since the sample is already a sulphate salt (the anion present, SO₄²⁻, does not react further with concentrated H₂SO₄ to release a gas).
Inference: Confirms absence of halide, nitrate, carbonate, or sulphite anions; consistent with SO₄²⁻ present.
d) Solution tests:
i) Add aqueous ammonia dropwise, then in excess:
Observation: A white gelatinous precipitate forms, which is insoluble in excess ammonia.
Inference: Confirms Al³⁺ present (Al(OH)₃ is amphoteric but does NOT dissolve in excess NH₃, unlike Zn²⁺ which does).
Equation: Al³⁺(aq) + 3NH₃(aq) + 3H₂O(l) → Al(OH)₃(s)↓ + 3NH₄⁺(aq)
ii) Add aqueous NaOH dropwise, then in excess:
Observation: A white gelatinous precipitate forms initially, which then dissolves in excess NaOH to give a colourless solution.
Inference: Confirms Al³⁺ present (amphoteric hydroxide, dissolves in excess NaOH to form the soluble aluminate ion).
Equations:
Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)↓
Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq) (excess)
iii) Add a few drops of aqueous barium chloride:
Observation: A white precipitate forms, insoluble in dilute HCl or HNO₃.
Inference: Confirms presence of SO₄²⁻ ion.
Equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)↓
iv) Add aqueous sodium tetraoxophosphate(V) solution dropwise and in excess; then add dilute ethanoic acid:
Observation: A white precipitate forms with sodium phosphate, which remains insoluble (or partially dissolves) on addition of dilute ethanoic acid.
Inference: Consistent with Al³⁺ forming aluminium phosphate (AlPO₄), which is insoluble in weak acids such as ethanoic acid (distinguishing from more soluble phosphates).
Equation: Al³⁺(aq) + PO₄³⁻(aq) → AlPO₄(s)↓
v) Add aqueous sodium trioxocarbonate(IV) solution dropwise and in excess:
Observation: A white precipitate forms with effervescence (gas bubbles, CO₂ evolved).
Inference: Al³⁺ salts, being salts of a weak base, hydrolyse Na₂CO₃ via mutual hydrolysis, releasing CO₂ gas and precipitating Al(OH)₃, rather than forming a stable normal carbonate (Al₂(CO₃)₃ does not exist as a stable compound in aqueous solution).
Equation: 2Al³⁺(aq) + 3CO₃²⁻(aq) + 3H₂O(l) → 2Al(OH)₃(s)↓ + 3CO₂(g)↑
e) Balanced equations for the positive tests observed in section (d):
i) Al³⁺(aq) + 3NH₃(aq) + 3H₂O(l) → Al(OH)₃(s) + 3NH₄⁺(aq)
ii) Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s); then Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq)
iii) Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
iv) Al³⁺(aq) + PO₄³⁻(aq) → AlPO₄(s)
v) 2Al³⁺(aq) + 3CO₃²⁻(aq) + 3H₂O(l) → 2Al(OH)₃(s) + 3CO₂(g)