2025 WAEC GCE CHEMISTRY 3 (PRACTICAL) ALTERNATIVE TO PRACTICAL QUESTIONS
QUESTION 1 – QUALITATIVE ANALYSIS (CATIONS & ANIONS)
You are provided with two salts, P and Q. Carry out the following tests and record your observations. Then state the correct inferences.
(a) Tests on Salt P
(i) Put a small portion of P in a test tube, add distilled water, and shake.
-
Record what you observe.
-
What does your observation indicate?
(ii) Add NaOH(aq) to a fresh sample of P dropwise, then in excess.
-
State the observations for both steps.
-
Identify the cation present.
(iii) Add NH₃(aq) to another portion dropwise and then in excess.
-
Record your observations.
-
State the cation present.
(b) Tests on Salt Q
(i) Dissolve a small amount of Q in water and add BaCl₂(aq) followed by HCl(aq).
-
Record the observations.
-
Identify the anion.
(ii) Dissolve another portion of Q in water and add AgNO₃(aq) followed by NH₃(aq).
-
Record the observations.
-
What anion is present?
Expected Answers (Marking Guide)
Salt P
-
(i) Observation: P dissolves to form a colourless solution.
Inference: A soluble salt. -
(ii) Observation with NaOH:
-
Dropwise: White gelatinous precipitate.
-
Excess: Precipitate dissolves.
Inference: Al³⁺ or Zn²⁺ likely present.
-
-
(iii) Observation with NH₃:
-
Dropwise: White precipitate.
-
Excess: Precipitate dissolves to give a clear solution.
Inference: Zn²⁺ confirmed.
-
Salt Q
-
(i) BaCl₂ + HCl test:
-
Observation: White precipitate forms with BaCl₂ but dissolves on addition of HCl.
Inference: Sulfite (SO₃²⁻) or carbonate originally present, but dissolving in acid confirms CO₃²⁻.
-
-
(ii) AgNO₃ + NH₃ test:
-
Observation: Cream precipitate forms with AgNO₃; partially dissolves in NH₃.
Inference: Bromide ion (Br⁻).
-
2025 WAEC GCE CHEMISTRY 3 (PRACTICAL) ALTERNATIVE TO PRACTICAL QUESTIONS
QUESTION 2 – VOLUMETRIC ANALYSIS (TITRATION)
You are provided with:
-
Solution A: 0.100 mol/dm³ sodium hydroxide (NaOH)
-
Solution B: A solution of hydrochloric acid (HCl) of unknown concentration
-
Indicator: Methyl orange
Using a pipette, measure 25.0 cm³ of B into a conical flask. Titrate against A until an end-point is reached.
Your titre values are recorded as:
-
24.8 cm³
-
25.0 cm³
-
24.9 cm³
(a) Calculate the average titre.
(b) Write the balanced equation for the reaction.
(c) Calculate the concentration of the acid B in mol/dm³.
(d) Calculate the concentration of B in g/dm³. (H = 1, Cl = 35.5)
Expected Answers (Marking Guide)
(a) Average titre:
24.8+25.0+24.93=24.9 cm3\frac{24.8 + 25.0 + 24.9}{3} = 24.9\ \text{cm}^3
(b) Balanced equation:
NaOH+HCl→NaCl+H2O\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}
(c) Concentration of HCl:
M1V1=M2V2\text{M}_1V_1 = \text{M}_2V_2 0.100×24.9=M2×25.00.100 \times 24.9 = M_2 \times 25.0 M2=0.0996 mol/dm3≈0.100 mol/dm3M_2 = 0.0996\ \text{mol/dm}^3 \approx 0.100\ \text{mol/dm}^3
(d) Concentration in g/dm³:
0.100×(36.5)=3.65 g/dm30.100 \times (36.5) = 3.65\ \text{g/dm}^3
2025 WAEC GCE CHEMISTRY 3 (PRACTICAL) ALTERNATIVE TO PRACTICAL QUESTIONS
QUESTION 3 – IDENTIFICATION OF ORGANIC COMPOUNDS
You are provided with an organic liquid X. Carry out the following tests and record your observations and inferences.
(a) Add Br₂(aq) to X and shake.
-
Observation:
-
Inference:
(b) Add acidified KMnO₄(aq) to another portion of X.
-
Observation:
-
Inference:
(c) Warm X with Fehling’s solution.
-
Observation:
-
Inference:
Expected Answers
(a) Bromine water test:
-
Observation: Bromine water is decolourised.
-
Inference: X is an unsaturated compound (alkene).
(b) Acidified KMnO₄:
-
Observation: Purple colour changes to colourless.
-
Inference: Presence of C=C double bond.
(c) Fehling’s solution:
-
Observation: No brick-red precipitate.
-
Inference: X is not an aldehyde; likely an alkene.
