WAEC SSCE - Chemistry - 2000 (Practical)

Credit will be given for strict adherence to 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 simple salts. Carry out the following exercises on C. Record your observations and identify any gases evolved. Sate the conclusion you draw from the result of each test.

(a) Put all of C into a beaker of boiling tube and add about \(10\text{cm}^3\) of distilled water. Stir well and filter. Keep the residue and keep the residue and the filtrate. Test the filtrate with litmus

(b) Add about \(1\text{ cm}^3\) of dilute hydrochloric acid to the residue in a test tube and warm gently Divide the reaction mixture into two portions

(c)(i) To the first portion from (b) add saturated sodiuim trioxocarbonate (V) solution in excess

(d) To bout \(2\text{cm}^2\) of the filtrate from (a) add a few drops of barium chloride solution followed by excess dilute hydrochloric acid

Jibu

Tests on Specimen C

Test	Observation	Inference / conclusion
(a) Add water to C, stir and filter. Test the filtrate with litmus paper.	Part of C dissolves, leaving a green insoluble residue. A colourless filtrate is obtained. The filtrate has no effect on red or blue litmus paper.	C contains a soluble salt and an insoluble salt. The soluble salt gives a neutral solution.
(b) Add dilute hydrochloric acid to the residue and warm gently.	Effervescence occurs. A colourless, odourless gas is evolved; it turns limewater milky. The green residue dissolves to give a blue-green solution.	The gas is carbon(IV) oxide, \(\mathrm{CO_2}\); hence \(\mathrm{CO_3^{2-}}\) is present. The blue-green solution indicates \(\mathrm{Cu^{2+}}\).
(c)(i) Add saturated sodium trioxocarbonate(V) solution in excess to one portion from (b).	Effervescence occurs at first. A blue-green precipitate is then formed and remains insoluble in excess sodium trioxocarbonate(V) solution.	The initial effervescence is due to reaction of excess hydrochloric acid with carbonate ions. The blue-green precipitate confirms \(\mathrm{Cu^{2+}}\) ions.
(d) Add barium chloride solution to the filtrate from (a), followed by excess dilute hydrochloric acid.	A white precipitate is formed. The precipitate does not dissolve in excess dilute hydrochloric acid.	Tetraoxosulphate(VI) ions, \(\mathrm{SO_4^{2-}}\), are present.

Equations:

\[\mathrm{CuCO_3(s)+2HCl(aq)\rightarrow CuCl_2(aq)+H_2O(l)+CO_2(g)}\]

\[\mathrm{Ba^{2+}(aq)+SO_4^{2-}(aq)\rightarrow BaSO_4(s)}\]

Final conclusion: C is a mixture of insoluble copper(II) trioxocarbonate(IV), \(\mathrm{CuCO_3}\), and a soluble tetraoxosulphate(VI), represented by sodium tetraoxosulphate(VI), \(\mathrm{Na_2SO_4}\).

Soma maelezo ya somo kwenye Kinetic Theory Of Matter And Gas Laws (JAMB)

Soma maelezo ya somo kwenye Oxidation And Reduction (JAMB)

Maelezo ya Majibu

Tests on Specimen C

Test	Observation	Inference / conclusion
(a) Add water to C, stir and filter. Test the filtrate with litmus paper.	Part of C dissolves, leaving a green insoluble residue. A colourless filtrate is obtained. The filtrate has no effect on red or blue litmus paper.	C contains a soluble salt and an insoluble salt. The soluble salt gives a neutral solution.
(b) Add dilute hydrochloric acid to the residue and warm gently.	Effervescence occurs. A colourless, odourless gas is evolved; it turns limewater milky. The green residue dissolves to give a blue-green solution.	The gas is carbon(IV) oxide, \(\mathrm{CO_2}\); hence \(\mathrm{CO_3^{2-}}\) is present. The blue-green solution indicates \(\mathrm{Cu^{2+}}\).
(c)(i) Add saturated sodium trioxocarbonate(V) solution in excess to one portion from (b).	Effervescence occurs at first. A blue-green precipitate is then formed and remains insoluble in excess sodium trioxocarbonate(V) solution.	The initial effervescence is due to reaction of excess hydrochloric acid with carbonate ions. The blue-green precipitate confirms \(\mathrm{Cu^{2+}}\) ions.
(d) Add barium chloride solution to the filtrate from (a), followed by excess dilute hydrochloric acid.	A white precipitate is formed. The precipitate does not dissolve in excess dilute hydrochloric acid.	Tetraoxosulphate(VI) ions, \(\mathrm{SO_4^{2-}}\), are present.

Equations:

\[\mathrm{CuCO_3(s)+2HCl(aq)\rightarrow CuCl_2(aq)+H_2O(l)+CO_2(g)}\]

\[\mathrm{Ba^{2+}(aq)+SO_4^{2-}(aq)\rightarrow BaSO_4(s)}\]

Final conclusion: C is a mixture of insoluble copper(II) trioxocarbonate(IV), \(\mathrm{CuCO_3}\), and a soluble tetraoxosulphate(VI), represented by sodium tetraoxosulphate(VI), \(\mathrm{Na_2SO_4}\).

Soma maelezo ya somo kwenye Kinetic Theory Of Matter And Gas Laws (JAMB)

Soma maelezo ya somo kwenye Oxidation And Reduction (JAMB)

Kinetic Theory Of Matter And Gas Laws

Oxidation And Reduction

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Swali 2 Ripoti

(a) Name one laboratory equipment used for

(i) keeping salts dry:

(ii) converting vapour to liquid during distillation

(ii) bubbling a gas into a liquid

(b) (i) What technique would you use to purity a sample of sodium chloride contaminated with ammonium chloride?

(i) Given sodium hydroxide solution, outline tne procedure you would use to determine whether or not all the ammonium chloride in (b)(i) above had been removed

(c) State what is observed on carrying out each of the following exercises.

(i) Adding few drops of methyl orange indicator to lime juice:

(ii) Adding few drops of concentrated \( \mathrm{HNO}_3 \) to acidified \( \mathrm{FeSO}_4 \) solution

(iii) Exposing a fresh precipitate of silver chloride to sunlight for 30 minutes

(iv) Adding zinc dust to dilute \( \mathrm{CuCl}_2 \) solution;

(v) Adding dilute \( \mathrm{H}_2\mathrm{SO}_4 \), to \( \mathrm{Pb}(\mathrm{NO}_3)_2 \) solution

Jibu

(a) Laboratory equipment

(i) Keeping salts dry: a desiccator (containing a drying agent such as anhydrous calcium chloride or silica gel).
(ii) Converting vapour to liquid during distillation: a (Liebig) condenser.
(iii) Bubbling a gas into a liquid: a gas-washing (Dreschel) bottle, or a delivery tube dipping into the liquid.

(b)(i) Purifying NaCl contaminated with NH₄Cl

Use sublimation. On gentle heating, ammonium chloride sublimes (turns directly to vapour and re-solidifies on a cool surface), leaving pure sodium chloride behind, because NaCl does not sublime.

(b)(ii) Testing whether all the NH₄Cl has been removed

Place a portion of the residue in a test tube, add sodium hydroxide solution and warm gently. Hold a piece of moist red litmus paper at the mouth of the tube.

If a pungent gas that turns moist red litmus blue (ammonia) is evolved, some ammonium chloride is still present.
If no ammonia smell and no colour change occur, all the ammonium chloride has been removed.

(c) Observations

Exercise	Observation
(i) Methyl orange added to lime juice	Indicator turns red/pink (lime juice is acidic).
(ii) Conc. HNO₃ added to acidified FeSO₄	Pale green solution turns yellow/brown as Fe²⁺ is oxidised to Fe³⁺ (a brown ring/colouration).
(iii) Fresh AgCl exposed to sunlight for 30 min	White precipitate gradually turns grey then black (photo-decomposition to silver metal).
(iv) Zinc dust added to dilute CuCl₂	Blue colour fades to colourless and a reddish-brown deposit of copper appears (Zn displaces Cu).
(v) Dilute H₂SO₄ added to Pb(NO₃)₂	A white precipitate of lead(II) tetraoxosulphate(VI), PbSO₄, forms.

Soma maelezo ya somo kwenye Solubility (JAMB)

Soma maelezo ya somo kwenye Acids, Bases And Salts (JAMB)

Soma maelezo ya somo kwenye Separation Of Mixtures And Purification Of Chemical Substances (JAMB)

Maelezo ya Majibu

(a) Laboratory equipment

(i) Keeping salts dry: a desiccator (containing a drying agent such as anhydrous calcium chloride or silica gel).
(ii) Converting vapour to liquid during distillation: a (Liebig) condenser.
(iii) Bubbling a gas into a liquid: a gas-washing (Dreschel) bottle, or a delivery tube dipping into the liquid.

(b)(i) Purifying NaCl contaminated with NH₄Cl

Use sublimation. On gentle heating, ammonium chloride sublimes (turns directly to vapour and re-solidifies on a cool surface), leaving pure sodium chloride behind, because NaCl does not sublime.

(b)(ii) Testing whether all the NH₄Cl has been removed

Place a portion of the residue in a test tube, add sodium hydroxide solution and warm gently. Hold a piece of moist red litmus paper at the mouth of the tube.

If a pungent gas that turns moist red litmus blue (ammonia) is evolved, some ammonium chloride is still present.
If no ammonia smell and no colour change occur, all the ammonium chloride has been removed.

(c) Observations

Exercise	Observation
(i) Methyl orange added to lime juice	Indicator turns red/pink (lime juice is acidic).
(ii) Conc. HNO₃ added to acidified FeSO₄	Pale green solution turns yellow/brown as Fe²⁺ is oxidised to Fe³⁺ (a brown ring/colouration).
(iii) Fresh AgCl exposed to sunlight for 30 min	White precipitate gradually turns grey then black (photo-decomposition to silver metal).
(iv) Zinc dust added to dilute CuCl₂	Blue colour fades to colourless and a reddish-brown deposit of copper appears (Zn displaces Cu).
(v) Dilute H₂SO₄ added to Pb(NO₃)₂	A white precipitate of lead(II) tetraoxosulphate(VI), PbSO₄, forms.

Soma maelezo ya somo kwenye Solubility (JAMB)

Soma maelezo ya somo kwenye Acids, Bases And Salts (JAMB)

Soma maelezo ya somo kwenye Separation Of Mixtures And Purification Of Chemical Substances (JAMB)

Solubility

Acids, Bases And Salts

Separation Of Mixtures And Purification Of Chemical Substances

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Swali 3 Ripoti

All your burette readings (initial and final), as well as the size of your pipette, must be recorded but on no account of experimental procedure is required. All calculations must be done in your answer book.

A is solution of trioxonitrate (V) acid, B is a solution containing 6.90 g of potassium trioxocarbonate (IV) per dm\(^3\)

(a) Put A into the buret and titrate it against 20.0 cm\(^3\) or 25.0 cm\(^3\) portions of B using methy orange or screened methyl orange as indicater. Record the volume of your pipette. Tabulate your burette readings and calculate the average volume A used. The equation for the reaction is \(\mathrm{K_2CO_{3(aq)} + 2HNO_{3(aq)} \to 2KNO_{3(aq)} + CO_{2(g)} + H_2O_{(l)}}\)

(b) From your results and the information provided calculate;

(i) concenytration of solution B in mol dm\(^{-3}\)

(ii) number of potassium ions in 1.00 dm\(^3\) of B [C = 12.0, O = 16.0, K = 39.0, Avogadro constant = 6.02 x 10\(^{23}\) mol \(^{-1}\)]

Jibu

Stoichiometry And Chemical Reactions

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