WAEC Chemistry 2026 Paper 3 - Quantitative and Qualitative Analysis Study Notes

General Instructions for Chemistry 2026 Paper 3 (Alternative B)

• Solution Identification:

  • Solution An: A solution of Hydrochloric Acid, $HCl$. The concentration is provided as $10.50\,cm^3$ of concentrated $HCl$ per $dm^3$ of solution.

  • Solution Bn: A solution of Sodium Hydroxide, $NaOH$. The concentration is provided as $4.00\,g$ of $NaOH$ per $dm^3$.

  • Solid Mixture Cn: A 1:1 intimate mixture consisting of Lead(II) nitrate ($Pb(NO_3)_2$) and Calcium chloride ($CaCl_2$).

• Accuracy and Assumptions:

  • Candidates must assume the actual concentrations of solutions A and B are exactly as stated in the instructions.

  • Burette readings must be recorded in a tabular format to exactly one decimal place ($0.1\,cm^3$ precision).

  • Working for all calculations must be shown clearly to receive full credit.

• Equipment Specifications:

  • Pipette Volume: Candidates are to use either a $20.0\,cm^3$ or $25.0\,cm^3$ pipette, as specified by the supervisor. This volume must be explicitly written in the answer booklet.

  • Calculators: The use of non-programmable calculators and mathematical tables is permitted.

• Evaluation Criteria:

  • Credit is awarded for strict adherence to instructions, the precision of qualitative observations, and the clarity of presented calculations.

Section A: Quantitative Analysis (Volumetric Analysis)

• Core Objective: To determine the concentration of the base, solution B ($NaOH$), by titrating it against the acid, solution A ($HCl$).

• Experimental Procedure:

  • Pipetting: Transfer exactly $25.0\,cm^3$ (or $20.0\,cm^3$ as per instruction) of the base solution B into a clean conical flask.

  • Burette Preparation: Fill the burette with the acid solution A ($HCl$).

  • Indicator: Add 2–3 drops of methyl orange indicator to the base in the conical flask.

  • Titration: Titrate solution B with solution A until the end point is reached. Record the initial and final burette readings in a table.

  • Average Volume: Calculate the average volume of acid used ($V_A$) from consistent titration runs.

• Required Calculations:

  • Concentration of B in $mol\,dm^{-3}$: Using the titration formula and the molar mass of $NaOH$.

  • Concentration of B in $g\,dm^{-3}$: Converting the molarity of solution B to mass concentration.

  • Neutralization Volume: Calculating the specific volume of acid solution A required to neutralize $250\,cm^3$ of base solution B.

  • Equivalent Molarity Comparison: Determining the concentration (in $g\,dm^{-3}$) of a $NaOH$ solution that contains the same number of moles as a $0.10\,mol\,dm^{-3}$ $H_2SO_4$ solution.

• Relevant Atomic Masses for Calculations:

  • Hydrogen ($H$) = $1.0$

  • Oxygen ($O$) = $16.0$

  • Sodium ($Na$) = $23.0$

  • Sulfur ($S$) = $32.0$

Section B: Qualitative Analysis (Test on Mixture C)

• Sample Composition: Mixture C contains two salts: Lead(II) nitrate ($Pb(NO_3)_2$) and Calcium chloride ($CaCl_2$) in a 1:1 ratio.

• Test (a): Preliminary Physical Examination

  • Task: Observe and record the physical appearance and color of solid mixture C.

• Test (b): Action of Heat

  • Task: Heat a small quantity of C strongly in a clean, dry test tube.

  • Chemistry Note: Lead(II) nitrate decomposes upon heating to produce lead(II) oxide (yellow when cold, reddish-brown when hot), oxygen gas ($O_2$), and nitrogen dioxide gas ($NO_2$, brown fumes).

• Test (c): Solubility Test

  • Task: Add approximately $5\,cm^3$ of distilled water to a spatulaful of C and shake thoroughly.

  • Chemistry Note: While $Pb(NO_3)_2$ and $CaCl_2$ are soluble, they react in solution to form $PbCl_2$, which is a white precipitate that is sparingly soluble in cold water but dissolves in hot water.

• Test (d): Analysis of Aqueous Solution

  • Preparation: Prepare a solution of C by dissolving a spatulaful in approximately $5\,cm^3$ of distilled water.

  • Sub-test (i): Reaction with Sodium Hydroxide ($NaOH_{(aq)}$):

    • Add $NaOH_{(aq)}$ in drops, then in excess to $2\,cm^3$ of the solution C.

    • Inference: Both $Pb^{2+}$ and $Ca^{2+}$ form white hydroxides. $Pb(OH)_2$ is amphoteric and dissolves in excess $NaOH$, while $Ca(OH)_2$ remains as a white precipitate (or is insoluble/slightly soluble).

  • Sub-test (ii): Reaction with Potassium Iodide ($KI$):

    • Add a few drops of $KI$ solution to $2\,cm^3$ of the solution C.

    • Inference: The presence of $Pb^{2+}$ ions results in the formation of a bright yellow precipitate of lead(II) iodide ($PbI_2$).

  • Sub-test (iii): Reaction with Sodium Sulfate ($Na_2SO_4$):

    • Add a few drops of $Na_2SO_4$ solution to $2\,cm^3$ of the solution C.

    • Inference: Formation of white precipitates such as $PbSO_4$ or $CaSO_4$.

  • Sub-test (iv): Reaction with Silver Nitrate ($AgNO_{3(aq)}$) and Ammonia ($NH_{3(aq)}$):

    • Add $AgNO_{3(aq)}$ to $2\,cm^3$ of solution C, then add dilute $NH_{3(aq)}$ in excess.

    • Inference: The presence of chloride ions ($Cl^-$) from $CaCl_2$ produces a white precipitate of $AgCl$, which dissolves in excess ammonia to form a colorless complex solution ($[Ag(NH_3)_2]^+$).

• Identification (e):

  • Final identification of the two salts as Lead(II) nitrate ($Pb(NO_3)_2$) and Calcium chloride ($CaCl_2$).