TEST OF PRACTICAL KNOWLEDGE QUESTION
All your burette readings (initials and final), as well as the size of your pipette, must be recorded but no account of experimental procedure is required. All calculations must be done in your answer booklet.
F is 2 mixture of two inorganic salts. Carry out the following exercises on F. Record your observations and identify any gas(es) evolved. State the conclusions you draw from the result of each test.
(a) Put all of F in a beaker and add about \(10\text{ cm}^3\) of distilled water. Stir well and filter. Keep the filtrate and the residue.
(b)(i) To about \(2\text{cm}^3\) of the filtrate. add \(\mathrm{NaOH}_{(aq)}\) in drops and then in excess.
(ii) To another \(2\text{cm}^3\) portion of the solution, add a few drops of \(\mathrm{NH3}_{(aq)}\) in drops and then in excess.
(c) To about \(2\text{cm}^3\) of the solution, add a few drops of \(\mathrm{HNO}_{3(aq)}\) followed by few drops of the drops of \(\mathrm{AgNO}_{3(aq)}\)
(d)(i) Put all the residue into a clean test-tube and add \(\mathrm{HNO}_{3(aq)}\)
(ii) To a portion of the solution from (d)(i)) add \(\mathrm{NaOH}_{(aq)}\) in drops and then in excess.
Qualitative analysis of F (a mixture of two inorganic salts)
The results of each test on F are recorded below. For each test the observation is stated and the inference (conclusion) drawn from it is given.
| Test | Observation | Inference |
|---|
| (a) All of F + about 10 cm\(^3\) distilled water; stir and filter. | Slight effervescence; a colourless, odourless gas is evolved. On filtering, a pale-blue (almost colourless) filtrate and a green residue are obtained. | F is a mixture of a soluble salt (in the filtrate) and an insoluble salt (the green residue). A copper(II) compound is indicated by the green residue and pale-blue filtrate. |
| (b)(i) Filtrate + NaOH\(_{(aq)}\) in drops, then in excess. | A white precipitate forms; it is insoluble in excess NaOH\(_{(aq)}\). A pale-blue precipitate is also present which remains insoluble in excess. | White, insoluble precipitate → Ca\(^{2+}\) (or Pb\(^{2+}\)); Ca\(^{2+}\) indicated. Pale-blue precipitate insoluble in excess → Cu\(^{2+}\) also present. |
| (b)(ii) Filtrate + NH\(_3{}_{(aq)}\) in drops, then in excess. | No white precipitate with Ca\(^{2+}\). A pale-blue precipitate forms which dissolves in excess to give a deep-blue solution. | No precipitate with ammonia confirms Ca\(^{2+}\) (not Pb\(^{2+}\)). Deep-blue solution in excess ammonia confirms Cu\(^{2+}\). |
| (c) Filtrate + HNO\(_{3(aq)}\), then AgNO\(_{3(aq)}\). | No gas evolved; a white precipitate forms. | Cl\(^{-}\) present (white AgCl). \[\text{Ag}^{+}_{(aq)} + \text{Cl}^{-}_{(aq)} \rightarrow \text{AgCl}_{(s)}\] |
| (d)(i) All of residue + HNO\(_{3(aq)}\). | Effervescence; a colourless, odourless gas is evolved that turns damp blue litmus paper red and turns lime water milky. The green residue dissolves to give a blue solution. | The gas is CO\(_2\); CO\(_3^{2-}\) (trioxocarbonate(IV)) present. \[\text{CO}_3^{2-} + 2\text{H}^{+} \rightarrow \text{H}_2\text{O} + \text{CO}_{2(g)}\] Blue solution → Cu\(^{2+}\) in the residue. |
| (d)(ii) Solution from (d)(i) + NaOH\(_{(aq)}\) in drops, then in excess. | A blue precipitate forms; it is insoluble in excess NaOH\(_{(aq)}\). | Cu\(^{2+}\) present (blue Cu(OH)\(_2\)). \[\text{Cu}^{2+}_{(aq)} + 2\text{OH}^{-}_{(aq)} \rightarrow \text{Cu(OH)}_{2(s)}\] |
Conclusion: F contains the cations Ca\(^{2+}\) and Cu\(^{2+}\) and the anions Cl\(^{-}\) and CO\(_3^{2-}\). The soluble portion (filtrate) supplies Ca\(^{2+}\) and Cl\(^{-}\) (calcium chloride, CaCl\(_2\)), while the green insoluble residue is a copper(II) trioxocarbonate(IV), CuCO\(_3\), which supplies Cu\(^{2+}\) and CO\(_3^{2-}\).
Qualitative analysis of F (a mixture of two inorganic salts)
The results of each test on F are recorded below. For each test the observation is stated and the inference (conclusion) drawn from it is given.
| Test | Observation | Inference |
|---|
| (a) All of F + about 10 cm\(^3\) distilled water; stir and filter. | Slight effervescence; a colourless, odourless gas is evolved. On filtering, a pale-blue (almost colourless) filtrate and a green residue are obtained. | F is a mixture of a soluble salt (in the filtrate) and an insoluble salt (the green residue). A copper(II) compound is indicated by the green residue and pale-blue filtrate. |
| (b)(i) Filtrate + NaOH\(_{(aq)}\) in drops, then in excess. | A white precipitate forms; it is insoluble in excess NaOH\(_{(aq)}\). A pale-blue precipitate is also present which remains insoluble in excess. | White, insoluble precipitate → Ca\(^{2+}\) (or Pb\(^{2+}\)); Ca\(^{2+}\) indicated. Pale-blue precipitate insoluble in excess → Cu\(^{2+}\) also present. |
| (b)(ii) Filtrate + NH\(_3{}_{(aq)}\) in drops, then in excess. | No white precipitate with Ca\(^{2+}\). A pale-blue precipitate forms which dissolves in excess to give a deep-blue solution. | No precipitate with ammonia confirms Ca\(^{2+}\) (not Pb\(^{2+}\)). Deep-blue solution in excess ammonia confirms Cu\(^{2+}\). |
| (c) Filtrate + HNO\(_{3(aq)}\), then AgNO\(_{3(aq)}\). | No gas evolved; a white precipitate forms. | Cl\(^{-}\) present (white AgCl). \[\text{Ag}^{+}_{(aq)} + \text{Cl}^{-}_{(aq)} \rightarrow \text{AgCl}_{(s)}\] |
| (d)(i) All of residue + HNO\(_{3(aq)}\). | Effervescence; a colourless, odourless gas is evolved that turns damp blue litmus paper red and turns lime water milky. The green residue dissolves to give a blue solution. | The gas is CO\(_2\); CO\(_3^{2-}\) (trioxocarbonate(IV)) present. \[\text{CO}_3^{2-} + 2\text{H}^{+} \rightarrow \text{H}_2\text{O} + \text{CO}_{2(g)}\] Blue solution → Cu\(^{2+}\) in the residue. |
| (d)(ii) Solution from (d)(i) + NaOH\(_{(aq)}\) in drops, then in excess. | A blue precipitate forms; it is insoluble in excess NaOH\(_{(aq)}\). | Cu\(^{2+}\) present (blue Cu(OH)\(_2\)). \[\text{Cu}^{2+}_{(aq)} + 2\text{OH}^{-}_{(aq)} \rightarrow \text{Cu(OH)}_{2(s)}\] |
Conclusion: F contains the cations Ca\(^{2+}\) and Cu\(^{2+}\) and the anions Cl\(^{-}\) and CO\(_3^{2-}\). The soluble portion (filtrate) supplies Ca\(^{2+}\) and Cl\(^{-}\) (calcium chloride, CaCl\(_2\)), while the green insoluble residue is a copper(II) trioxocarbonate(IV), CuCO\(_3\), which supplies Cu\(^{2+}\) and CO\(_3^{2-}\).
