Credit will be given for strict adherence to the... | WAEC SSCE - Chemistry - 2014 (Practical)

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Credit will be given for strict adherence to the instructions, for observations, precisely recorded, and for accurate inferences. All tests, observations, and inferences must be clearly entered in your answer book, in ink, at the time they are made.

C contains two cations and two anions. Perform the following exercises on C. Record your observations and identify any gas (es) evolved. State the conclusion you draw from the result of each test.

(a) Dissolve all of C in about 10 cm\(^3\) of distilled water. Stir the resulting solution thoroughly.

(i) To about 2 cm\(^3\) of the solution, add few drops of AgNO\(_3\) solution, followed by HNO\(_{3(aq)}\). To the mixture, add excess NH\(_{3(aq)}\)

(ii) To another 2 cm\(^3\) portion of the solution, add dil. HCl followed by BaCl\(_2\) solution.

(iii) To another 2 cm\(^3\) portion of the solution, add NaOH\(_{3(aq)}\) dropwise and then in excess. Warm the mixture.

(iv) To another 2 cm\(^3\) portion of the solution, add NH\(_{3(aq)}\) dropwise and then in excess.

Answer

Result for C

All of C dissolves in about 10 cm³ of distilled water to give a clear solution.

Test	Observation	Inference / conclusion
(i) To 2 cm³ of C(aq), add AgNO₃(aq), followed by HNO₃(aq), then excess NH₃(aq).	A white precipitate forms with AgNO₃(aq). The precipitate is insoluble in dilute HNO₃(aq), but dissolves in excess NH₃(aq). No gas is evolved.	Chloride ion, Cl⁻, is present. The precipitate is AgCl.
(ii) To another 2 cm³ portion, add dilute HCl(aq), followed by BaCl₂(aq).	No visible reaction and no gas is evolved on adding dilute HCl. A white chalky precipitate forms on adding BaCl₂(aq).	Sulphate ion, SO₄²⁻, is present. The precipitate is BaSO₄.
(iii) To another 2 cm³ portion, add NaOH(aq) dropwise, then in excess. Warm the mixture.	A white gelatinous precipitate forms on adding NaOH(aq) dropwise. It dissolves in excess NaOH(aq). On warming, a colourless pungent gas is evolved. The gas turns damp red litmus paper blue and forms dense white fumes with hydrogen chloride gas.	Zn²⁺ or Al³⁺ is initially suspected from the amphoteric white precipitate. The gas is ammonia, NH₃; hence NH₄⁺ is present.
(iv) To another 2 cm³ portion, add NH₃(aq) dropwise, then in excess.	A white gelatinous precipitate forms on adding NH₃(aq) dropwise. The precipitate dissolves in excess NH₃(aq).	Zn²⁺ is present. Zinc hydroxide dissolves in excess ammonia to form a colourless complex.

Final conclusion: C contains the cations Zn²⁺ and NH₄⁺, and the anions Cl⁻ and SO₄²⁻.

Read lesson note on Metals And Their Compounds (JAMB)

Read lesson note on Acids, Bases And Salts (JAMB)

Answer Details

Result for C

All of C dissolves in about 10 cm³ of distilled water to give a clear solution.

Test	Observation	Inference / conclusion
(i) To 2 cm³ of C(aq), add AgNO₃(aq), followed by HNO₃(aq), then excess NH₃(aq).	A white precipitate forms with AgNO₃(aq). The precipitate is insoluble in dilute HNO₃(aq), but dissolves in excess NH₃(aq). No gas is evolved.	Chloride ion, Cl⁻, is present. The precipitate is AgCl.
(ii) To another 2 cm³ portion, add dilute HCl(aq), followed by BaCl₂(aq).	No visible reaction and no gas is evolved on adding dilute HCl. A white chalky precipitate forms on adding BaCl₂(aq).	Sulphate ion, SO₄²⁻, is present. The precipitate is BaSO₄.
(iii) To another 2 cm³ portion, add NaOH(aq) dropwise, then in excess. Warm the mixture.	A white gelatinous precipitate forms on adding NaOH(aq) dropwise. It dissolves in excess NaOH(aq). On warming, a colourless pungent gas is evolved. The gas turns damp red litmus paper blue and forms dense white fumes with hydrogen chloride gas.	Zn²⁺ or Al³⁺ is initially suspected from the amphoteric white precipitate. The gas is ammonia, NH₃; hence NH₄⁺ is present.
(iv) To another 2 cm³ portion, add NH₃(aq) dropwise, then in excess.	A white gelatinous precipitate forms on adding NH₃(aq) dropwise. The precipitate dissolves in excess NH₃(aq).	Zn²⁺ is present. Zinc hydroxide dissolves in excess ammonia to form a colourless complex.

Final conclusion: C contains the cations Zn²⁺ and NH₄⁺, and the anions Cl⁻ and SO₄²⁻.

Read lesson note on Metals And Their Compounds (JAMB)

Read lesson note on Acids, Bases And Salts (JAMB)

Metals And Their Compounds

Acids, Bases And Salts

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