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), FeSO4; D is zinc trioxocarbonate(IV), ZnCO3.
(a)(i) C dissolves to give a pale green solution; litmus shows it is faintly acidic. Inference: Fe2+ salt of a strong acid.
(a)(ii) First portion + NaOH (drops then excess): a dirty green precipitate of Fe(OH)2 forms, insoluble in excess (turning brown on standing as it oxidizes). Inference: Fe2+ present.
\[Fe^{2+} + 2OH^- \rightarrow Fe(OH)_2\]
(a)(iii) Second portion + BaCl2 then dilute HCl in excess: a white precipitate (BaSO4) forms that is insoluble in the excess dilute HCl. Inference: SO42- (sulphate) present.
(b)(i) Half of D heated strongly: a colourless gas is evolved that turns limewater milky (carbon(IV) oxide); the residue is yellow when hot and white when cold (ZnO). Inference: trioxocarbonate(IV) present; residue is zinc oxide.
\[ZnCO_3 \rightarrow ZnO + CO_2\]
(b)(ii) Residue + dilute HCl: the residue dissolves to give a colourless solution. \[ZnO + 2HCl \rightarrow ZnCl_2 + H_2O\]
(b)(iii) First portion + NaOH (drops then excess): a white gelatinous precipitate of Zn(OH)2 forms that is soluble in excess NaOH giving a colourless solution. Inference: Zn2+ present.
(b)(iv) Second portion + aqueous ammonia (drops then excess): a white precipitate of Zn(OH)2 forms that is soluble in excess ammonia. Inference: Zn2+ confirmed.
C is iron(II) tetraoxosulphate(VI), FeSO4; D is zinc trioxocarbonate(IV), ZnCO3.
(a)(i) C dissolves to give a pale green solution; litmus shows it is faintly acidic. Inference: Fe2+ salt of a strong acid.
(a)(ii) First portion + NaOH (drops then excess): a dirty green precipitate of Fe(OH)2 forms, insoluble in excess (turning brown on standing as it oxidizes). Inference: Fe2+ present.
\[Fe^{2+} + 2OH^- \rightarrow Fe(OH)_2\]
(a)(iii) Second portion + BaCl2 then dilute HCl in excess: a white precipitate (BaSO4) forms that is insoluble in the excess dilute HCl. Inference: SO42- (sulphate) present.
(b)(i) Half of D heated strongly: a colourless gas is evolved that turns limewater milky (carbon(IV) oxide); the residue is yellow when hot and white when cold (ZnO). Inference: trioxocarbonate(IV) present; residue is zinc oxide.
\[ZnCO_3 \rightarrow ZnO + CO_2\]
(b)(ii) Residue + dilute HCl: the residue dissolves to give a colourless solution. \[ZnO + 2HCl \rightarrow ZnCl_2 + H_2O\]
(b)(iii) First portion + NaOH (drops then excess): a white gelatinous precipitate of Zn(OH)2 forms that is soluble in excess NaOH giving a colourless solution. Inference: Zn2+ present.
(b)(iv) Second portion + aqueous ammonia (drops then excess): a white precipitate of Zn(OH)2 forms that is soluble in excess ammonia. Inference: Zn2+ confirmed.
