Credit will be given for strict adherence to the... | WAEC SSCE - Chemistry - 2013 (Practical)

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

C is a double salt. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion drawn from the result of each test.

a) Put all of C into a test tube. Add about 5cm\(^3\) of distilled water, stir and test with litmus paper. Divide the Solution into two portions

(b) To the first portion, add sodium hydroxide solution in drops and then in excess. Heat the resulting mixture and keep it for minutes.

Answer

Test	Observation	Inference
(a) All of C + about 5 cm³ distilled water, stir, test with litmus	C dissolves completely to give a light green solution; the solution turns blue litmus paper red	C is a soluble salt; the solution is acidic
(b) First portion + NaOH_(aq) dropwise then in excess, then heat and keep for a few minutes	A green gelatinous precipitate forms which is insoluble in excess NaOH; on heating a colourless, pungent, choking gas is evolved which turns damp red litmus paper blue and gives dense white fumes with conc. HCl; the precipitate slowly turns brown at the upper part of the tube	Fe²⁺ present (green Fe(OH)₂); NH₄⁺ present (ammonia gas from the ammonium ion); the Fe²⁺ is oxidized to Fe³⁺ in air, giving the brown colour
(c) Second portion + a few drops of BaCl_2(aq), then excess dilute HCl	A white precipitate forms which does not dissolve in excess dilute HCl	SO₄^2- present (white BaSO₄, insoluble in acid)

Test	Observation	Inference
(a) All of C + about 5 cm³ distilled water, stir, test with litmus	C dissolves completely to give a light green solution; the solution turns blue litmus paper red	C is a soluble salt; the solution is acidic
(b) First portion + NaOH_(aq) dropwise then in excess, then heat and keep for a few minutes	A green gelatinous precipitate forms which is insoluble in excess NaOH; on heating a colourless, pungent, choking gas is evolved which turns damp red litmus paper blue and gives dense white fumes with conc. HCl; the precipitate slowly turns brown at the upper part of the tube	Fe²⁺ present (green Fe(OH)₂); NH₄⁺ present (ammonia gas from the ammonium ion); the Fe²⁺ is oxidized to Fe³⁺ in air, giving the brown colour
(c) Second portion + a few drops of BaCl_2(aq), then excess dilute HCl	A white precipitate forms which does not dissolve in excess dilute HCl	SO₄^2- present (white BaSO₄, insoluble in acid)

Acids, Bases And Salts

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