TEST OF PRACTICAL KNOWLEDGE QUESTION
Credit will be given for strict adherere 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 sample of iron (ii) tetraoxosulphate (VI). D is a sample of zinc trioxocarbonate (IV). Carry out the following exercises on C and D. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test.
(a)(i) Put all of C in a test tube and add about 5 cm\(^3\) of distilled water. Stir and test with litmus paper. Divide the solution into two portions
(ii) To the first portion, add sodium hydroxide solution in drops and then in excess.
(iii) To the second portion, add few drops of barium chloride solution, followed by dilute hydrochloric acid in excess.
(b)(i) Put half of D in a dry test tube and heat strongly. Allow to cool.
(ii) Add about 5 cm\(^3\) of dilute hydrochloric acid to the residue and divide the solution into two portions.
(iii) To the first portion, add sodium hydroxide solution in drops and then in excess.
(iv) To the second portion, add aqueous ammonia in drops and then in excess.
C is iron(II) tetraoxosulphate(VI), \(\text{FeSO}_4\); D is zinc trioxocarbonate(IV), \(\text{ZnCO}_3\). The results of the tests are recorded below.
| Test |
Observation |
Inference |
| (a)(i) All of C + about 5 cm\(^3\) distilled water; stir and test with litmus paper. |
C dissolves completely to give a pale (very faint green) solution; blue litmus paper turns red, red litmus unchanged. |
C is a soluble salt; the solution is acidic (salt of a weak base and a strong acid, \(\text{Fe}^{2+}\) undergoing hydrolysis). |
| (a)(ii) First portion + \(\text{NaOH}_{(aq)}\) in drops, then in excess. |
A dirty-green gelatinous precipitate forms; the precipitate is insoluble in excess \(\text{NaOH}\). |
\(\text{Fe}^{2+}\) present. \[\text{Fe}^{2+}_{(aq)} + 2\text{OH}^-_{(aq)} \rightarrow \text{Fe(OH)}_{2(s)}\] |
| (a)(iii) Second portion + a few drops of \(\text{BaCl}_{2(aq)}\), then dilute \(\text{HCl}\) in excess. |
A white precipitate forms; the precipitate is insoluble in excess dilute \(\text{HCl}\). |
\(\text{SO}_4^{2-}\) present. \[\text{Ba}^{2+}_{(aq)} + \text{SO}_4^{2-}_{(aq)} \rightarrow \text{BaSO}_{4(s)}\] |
| (b)(i) Half of D in a dry test tube, heated strongly, then allowed to cool. |
The white solid turns yellow on strong heating and returns to white on cooling; a colourless, odourless gas is evolved which turns lime water milky. |
D is a trioxocarbonate(IV) (carbonate); \(\text{CO}_2\) evolved; residue is \(\text{ZnO}\) (yellow when hot, white when cold). \[\text{ZnCO}_{3(s)} \rightarrow \text{ZnO}_{(s)} + \text{CO}_{2(g)}\] |
| (b)(ii) Residue + about 5 cm\(^3\) dilute \(\text{HCl}\); divide into two portions. |
A little effervescence (from unchanged carbonate); the residue dissolves to give a colourless solution. |
\(\text{ZnO}\)/residual \(\text{ZnCO}_3\) present giving \(\text{Zn}^{2+}\). \[\text{ZnO}_{(s)} + 2\text{HCl}_{(aq)} \rightarrow \text{ZnCl}_{2(aq)} + \text{H}_2\text{O}_{(l)}\] |
| (b)(iii) First portion + \(\text{NaOH}_{(aq)}\) in drops, then in excess. |
A white gelatinous precipitate forms; the precipitate dissolves (is soluble) in excess \(\text{NaOH}\). |
\(\text{Zn}^{2+}\) present (amphoteric hydroxide). \[\text{Zn}^{2+}_{(aq)} + 2\text{OH}^-_{(aq)} \rightarrow \text{Zn(OH)}_{2(s)}\] \[\text{Zn(OH)}_{2(s)} + 2\text{OH}^-_{(aq)} \rightarrow \text{ZnO}_2^{2-}_{(aq)} + 2\text{H}_2\text{O}_{(l)}\] |
| (b)(iv) Second portion + aqueous ammonia in drops, then in excess. |
A white gelatinous precipitate forms; the precipitate dissolves (is soluble) in excess aqueous ammonia. |
\(\text{Zn}^{2+}\) confirmed (soluble complex formed). \[\text{Zn(OH)}_{2(s)} + 4\text{NH}_{3(aq)} \rightarrow [\text{Zn(NH}_3)_4]^{2+}_{(aq)} + 2\text{OH}^-_{(aq)}\] |
Overall conclusion: C contains \(\text{Fe}^{2+}\) and \(\text{SO}_4^{2-}\) ions and is confirmed as iron(II) tetraoxosulphate(VI), \(\text{FeSO}_4\). D is a trioxocarbonate(IV) that on heating gives off \(\text{CO}_2\) and leaves \(\text{ZnO}\); its solution contains \(\text{Zn}^{2+}\), confirming D as zinc trioxocarbonate(IV), \(\text{ZnCO}_3\).
C is iron(II) tetraoxosulphate(VI), \(\text{FeSO}_4\); D is zinc trioxocarbonate(IV), \(\text{ZnCO}_3\). The results of the tests are recorded below.
| Test |
Observation |
Inference |
| (a)(i) All of C + about 5 cm\(^3\) distilled water; stir and test with litmus paper. |
C dissolves completely to give a pale (very faint green) solution; blue litmus paper turns red, red litmus unchanged. |
C is a soluble salt; the solution is acidic (salt of a weak base and a strong acid, \(\text{Fe}^{2+}\) undergoing hydrolysis). |
| (a)(ii) First portion + \(\text{NaOH}_{(aq)}\) in drops, then in excess. |
A dirty-green gelatinous precipitate forms; the precipitate is insoluble in excess \(\text{NaOH}\). |
\(\text{Fe}^{2+}\) present. \[\text{Fe}^{2+}_{(aq)} + 2\text{OH}^-_{(aq)} \rightarrow \text{Fe(OH)}_{2(s)}\] |
| (a)(iii) Second portion + a few drops of \(\text{BaCl}_{2(aq)}\), then dilute \(\text{HCl}\) in excess. |
A white precipitate forms; the precipitate is insoluble in excess dilute \(\text{HCl}\). |
\(\text{SO}_4^{2-}\) present. \[\text{Ba}^{2+}_{(aq)} + \text{SO}_4^{2-}_{(aq)} \rightarrow \text{BaSO}_{4(s)}\] |
| (b)(i) Half of D in a dry test tube, heated strongly, then allowed to cool. |
The white solid turns yellow on strong heating and returns to white on cooling; a colourless, odourless gas is evolved which turns lime water milky. |
D is a trioxocarbonate(IV) (carbonate); \(\text{CO}_2\) evolved; residue is \(\text{ZnO}\) (yellow when hot, white when cold). \[\text{ZnCO}_{3(s)} \rightarrow \text{ZnO}_{(s)} + \text{CO}_{2(g)}\] |
| (b)(ii) Residue + about 5 cm\(^3\) dilute \(\text{HCl}\); divide into two portions. |
A little effervescence (from unchanged carbonate); the residue dissolves to give a colourless solution. |
\(\text{ZnO}\)/residual \(\text{ZnCO}_3\) present giving \(\text{Zn}^{2+}\). \[\text{ZnO}_{(s)} + 2\text{HCl}_{(aq)} \rightarrow \text{ZnCl}_{2(aq)} + \text{H}_2\text{O}_{(l)}\] |
| (b)(iii) First portion + \(\text{NaOH}_{(aq)}\) in drops, then in excess. |
A white gelatinous precipitate forms; the precipitate dissolves (is soluble) in excess \(\text{NaOH}\). |
\(\text{Zn}^{2+}\) present (amphoteric hydroxide). \[\text{Zn}^{2+}_{(aq)} + 2\text{OH}^-_{(aq)} \rightarrow \text{Zn(OH)}_{2(s)}\] \[\text{Zn(OH)}_{2(s)} + 2\text{OH}^-_{(aq)} \rightarrow \text{ZnO}_2^{2-}_{(aq)} + 2\text{H}_2\text{O}_{(l)}\] |
| (b)(iv) Second portion + aqueous ammonia in drops, then in excess. |
A white gelatinous precipitate forms; the precipitate dissolves (is soluble) in excess aqueous ammonia. |
\(\text{Zn}^{2+}\) confirmed (soluble complex formed). \[\text{Zn(OH)}_{2(s)} + 4\text{NH}_{3(aq)} \rightarrow [\text{Zn(NH}_3)_4]^{2+}_{(aq)} + 2\text{OH}^-_{(aq)}\] |
Overall conclusion: C contains \(\text{Fe}^{2+}\) and \(\text{SO}_4^{2-}\) ions and is confirmed as iron(II) tetraoxosulphate(VI), \(\text{FeSO}_4\). D is a trioxocarbonate(IV) that on heating gives off \(\text{CO}_2\) and leaves \(\text{ZnO}\); its solution contains \(\text{Zn}^{2+}\), confirming D as zinc trioxocarbonate(IV), \(\text{ZnCO}_3\).
