TEST OF PRACTICAL KNOWLEDGE QUESTION Credit will... | WAEC SSCE - Chemistry - 2007

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TEST OF PRACTICAL KNOWLEDGE QUESTION

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 is a mixture of two inorganic salts. Carry out 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) Put all of C in a test tube and add about \(10\ \text{cm}^3\) of distilled water. Stir, filter, and keep the filtrate and the residue.

(b) Put the residue into a test tube and add about \(5\ \text{cm}^3\) of dilute HCl. Shake to dissolve

(i) To about \(2\ \text{cm}^3\) of the solution, add \(\mathrm{NaOH_{(aq)}}\) in drops and then in excess

(ii) To another \(2\ \text{cm}^3\) portion of the solution, add \(\mathrm{NH_{3(aq)}}\) in drops and then in excess.

(c) To about \(2\ \text{cm}^3\) portion of the filtrate, add few drops of dilute \(\mathrm{HNO_3}\), and then \(\mathrm{AgNO_{3(aq)}}\) a followed by aqueous \(\mathrm{NH_3}\) in excess.

Answer

Test	Observation	Inference
(a) Add about 10 cm³ of distilled water to all of C, stir and filter.	C dissolves partly. A white residue remains on the filter paper and a colourless filtrate is obtained.	C contains a water-soluble salt and a water-insoluble white salt.
(b) Add dilute HCl to the residue and shake.	Effervescence occurs and the residue dissolves, giving a colourless solution. The colourless gas turns limewater milky.	The gas is carbon dioxide, CO₂. A carbonate ion, CO₃²⁻, is present in the insoluble salt.
(b)(i) To 2 cm³ of the solution from (b), add NaOH_(aq) dropwise and then in excess.	A white gelatinous precipitate forms. The precipitate dissolves in excess NaOH_(aq).	Zn²⁺, Al³⁺ or Pb²⁺ may be present.
(b)(ii) To a fresh 2 cm³ portion of the solution from (b), add NH_3(aq) dropwise and then in excess.	A white gelatinous precipitate forms. The precipitate dissolves in excess aqueous ammonia.	Zn²⁺ is confirmed.
(c) To 2 cm³ of the filtrate, add dilute HNO₃, then AgNO_3(aq), followed by excess NH_3(aq).	There is no visible reaction with dilute HNO₃. A white precipitate forms on adding AgNO_3(aq); it dissolves in excess NH_3(aq).	Cl⁻ is present and is confirmed.

Test	Observation	Inference
(a) Add about 10 cm³ of distilled water to all of C, stir and filter.	C dissolves partly. A white residue remains on the filter paper and a colourless filtrate is obtained.	C contains a water-soluble salt and a water-insoluble white salt.
(b) Add dilute HCl to the residue and shake.	Effervescence occurs and the residue dissolves, giving a colourless solution. The colourless gas turns limewater milky.	The gas is carbon dioxide, CO₂. A carbonate ion, CO₃²⁻, is present in the insoluble salt.
(b)(i) To 2 cm³ of the solution from (b), add NaOH_(aq) dropwise and then in excess.	A white gelatinous precipitate forms. The precipitate dissolves in excess NaOH_(aq).	Zn²⁺, Al³⁺ or Pb²⁺ may be present.
(b)(ii) To a fresh 2 cm³ portion of the solution from (b), add NH_3(aq) dropwise and then in excess.	A white gelatinous precipitate forms. The precipitate dissolves in excess aqueous ammonia.	Zn²⁺ is confirmed.
(c) To 2 cm³ of the filtrate, add dilute HNO₃, then AgNO_3(aq), followed by excess NH_3(aq).	There is no visible reaction with dilute HNO₃. A white precipitate forms on adding AgNO_3(aq); it dissolves in excess NH_3(aq).	Cl⁻ is present and is confirmed.

Chemical Combination

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