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 e...
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
C is a mıxture of two salts, containing one cation and two anions. 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 beaker and add about \(10\ \text{cm}^3\) of distilled water. Stir well and filter. Keep the filtrate and the residue.
(b) To about \(2\ \text{cm}^3\) of the filtrate, add few drops of \(\mathrm{AgNO}_{(aq)}\), followed by \(\mathrm{HNO}_{3(g)}\). Add excess \(\mathrm{NH}_{3(aq)}\) to the resulting mixture.
(c)(i) Put all of the residue into a clean test tube and add about 5 cm of \(\mathrm{HNO}_{3(aq)}\)
(ii) To about \(2\ \text{cm}^3\) of the solution from 2(c)(i), add \(\mathrm{NaOH}_{(aq)}\) in drops and then in excess
(iii) To another \(2\ \text{cm}^3\) of the solution from 2(c)(i), add \(\mathrm{NH}_{3(aq)}\) in drops and then in excess.
Specimen C is a mixture of two salts sharing one cation, Zn2+, and containing two anions: the soluble part supplies chloride, Cl−, and the insoluble residue supplies carbonate, CO32−. That is, C is a mixture of zinc chloride and zinc carbonate.
The observations and inferences for each test are recorded below.
Test
Observation
Inference
(a) All of C placed in a beaker, about 10 cm3 distilled water added, stirred and filtered.
C partly dissolves; a colourless filtrate is obtained and a residue remains on the filter paper.
C is a mixture of a soluble salt (in the filtrate) and an insoluble salt (the residue).
(b) To about 2 cm3 of the filtrate, a few drops of AgNO3(aq) are added, followed by dilute HNO3(aq), then excess NH3(aq).
A white precipitate forms; it is insoluble in dilute HNO3; it dissolves completely in excess aqueous ammonia.
Cl− present in the soluble salt. \(\text{Ag}^{+} + \text{Cl}^{-} \rightarrow \text{AgCl}\downarrow\) (white), and \(\text{AgCl} + 2\text{NH}_3 \rightarrow [\text{Ag(NH}_3)_2]^{+} + \text{Cl}^{-}\).
(c)(i) All of the residue is put into a clean test tube and about 5 cm3 of dilute HNO3(aq) is added.
Brisk effervescence of a colourless, odourless gas that turns lime water milky; the residue dissolves to a colourless solution.
Gas is CO2; CO32− present in the insoluble salt. \(\text{CO}_3^{2-} + 2\text{H}^{+} \rightarrow \text{H}_2\text{O} + \text{CO}_2\uparrow\).
(c)(ii) To about 2 cm3 of the solution from (c)(i), NaOH(aq) is added in drops and then in excess.
A white gelatinous precipitate forms which dissolves in excess NaOH to give a colourless solution.
Zn2+ or Al3+ present. \(\text{Zn}^{2+} + 2\text{OH}^{-} \rightarrow \text{Zn(OH)}_2\downarrow\), soluble in excess as \([\text{Zn(OH)}_4]^{2-}\).
(c)(iii) To a further 2 cm3 of the solution from (c)(i), NH3(aq) is added in drops and then in excess.
A white gelatinous precipitate forms which dissolves in excess aqueous ammonia to give a colourless solution.
Zn2+ confirmed; Al3+ excluded, since Al(OH)3 is insoluble in excess NH3. \(\text{Zn(OH)}_2 + 4\text{NH}_3 \rightarrow [\text{Zn(NH}_3)_4]^{2+} + 2\text{OH}^{-}\).
Conclusion: The single cation in C is Zn2+ (given by (c)(ii) and confirmed by its dissolving in excess NH3 in (c)(iii)). The two anions are chloride, Cl−, in the soluble portion (test (b)) and carbonate, CO32−, in the insoluble residue (test (c)(i)). Therefore C is a mixture of zinc chloride, ZnCl2, and zinc carbonate, ZnCO3.
Specimen C is a mixture of two salts sharing one cation, Zn2+, and containing two anions: the soluble part supplies chloride, Cl−, and the insoluble residue supplies carbonate, CO32−. That is, C is a mixture of zinc chloride and zinc carbonate.
The observations and inferences for each test are recorded below.
Test
Observation
Inference
(a) All of C placed in a beaker, about 10 cm3 distilled water added, stirred and filtered.
C partly dissolves; a colourless filtrate is obtained and a residue remains on the filter paper.
C is a mixture of a soluble salt (in the filtrate) and an insoluble salt (the residue).
(b) To about 2 cm3 of the filtrate, a few drops of AgNO3(aq) are added, followed by dilute HNO3(aq), then excess NH3(aq).
A white precipitate forms; it is insoluble in dilute HNO3; it dissolves completely in excess aqueous ammonia.
Cl− present in the soluble salt. \(\text{Ag}^{+} + \text{Cl}^{-} \rightarrow \text{AgCl}\downarrow\) (white), and \(\text{AgCl} + 2\text{NH}_3 \rightarrow [\text{Ag(NH}_3)_2]^{+} + \text{Cl}^{-}\).
(c)(i) All of the residue is put into a clean test tube and about 5 cm3 of dilute HNO3(aq) is added.
Brisk effervescence of a colourless, odourless gas that turns lime water milky; the residue dissolves to a colourless solution.
Gas is CO2; CO32− present in the insoluble salt. \(\text{CO}_3^{2-} + 2\text{H}^{+} \rightarrow \text{H}_2\text{O} + \text{CO}_2\uparrow\).
(c)(ii) To about 2 cm3 of the solution from (c)(i), NaOH(aq) is added in drops and then in excess.
A white gelatinous precipitate forms which dissolves in excess NaOH to give a colourless solution.
Zn2+ or Al3+ present. \(\text{Zn}^{2+} + 2\text{OH}^{-} \rightarrow \text{Zn(OH)}_2\downarrow\), soluble in excess as \([\text{Zn(OH)}_4]^{2-}\).
(c)(iii) To a further 2 cm3 of the solution from (c)(i), NH3(aq) is added in drops and then in excess.
A white gelatinous precipitate forms which dissolves in excess aqueous ammonia to give a colourless solution.
Zn2+ confirmed; Al3+ excluded, since Al(OH)3 is insoluble in excess NH3. \(\text{Zn(OH)}_2 + 4\text{NH}_3 \rightarrow [\text{Zn(NH}_3)_4]^{2+} + 2\text{OH}^{-}\).
Conclusion: The single cation in C is Zn2+ (given by (c)(ii) and confirmed by its dissolving in excess NH3 in (c)(iii)). The two anions are chloride, Cl−, in the soluble portion (test (b)) and carbonate, CO32−, in the insoluble residue (test (c)(i)). Therefore C is a mixture of zinc chloride, ZnCl2, and zinc carbonate, ZnCO3.
