TEST OF PRACTICAL KNOWLEDGE QUESTION
Credit will be given for strict adherence to the instructions, for observations precisely recorded, and for accurate inferences. AIl tests. observations and inferences must be clearly entered in your answer book, in ink, at the time they are made.
C is a double salt. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion drawn from the result of each test.
a) Put all of C into a test tube. Add about 5cm\(^3\) of distilled water, stir and test with litmus paper. Divide the Solution into two portions
(b) To the first portion, add sodium hydroxide solution in drops and then in excess. Heat the resulting mixture and keep it for minutes.
(c) To the second portion, add few drops of Bacl\(_{2(aq)}\) followed by excess dilute hydrochloric acid
| Test | Observation | Inference |
|---|
| (a) All of C + about 5 cm3 distilled water, stir, test with litmus | C dissolves completely to give a light green solution; the solution turns blue litmus paper red | C is a soluble salt; the solution is acidic |
| (b) First portion + NaOH(aq) dropwise then in excess, then heat and keep for a few minutes | A green gelatinous precipitate forms which is insoluble in excess NaOH; on heating a colourless, pungent, choking gas is evolved which turns damp red litmus paper blue and gives dense white fumes with conc. HCl; the precipitate slowly turns brown at the upper part of the tube | Fe2+ present (green Fe(OH)2); NH4+ present (ammonia gas from the ammonium ion); the Fe2+ is oxidized to Fe3+ in air, giving the brown colour |
| (c) Second portion + a few drops of BaCl2(aq), then excess dilute HCl | A white precipitate forms which does not dissolve in excess dilute HCl | SO42- present (white BaSO4, insoluble in acid) |
Conclusion: C contains Fe2+, NH4+ and SO42- ions, so C is the double salt ammonium iron(II) tetraoxosulphate(VI), \((NH_4)_2SO_4 \cdot FeSO_4 \cdot 6H_2O\).
| Test | Observation | Inference |
|---|
| (a) All of C + about 5 cm3 distilled water, stir, test with litmus | C dissolves completely to give a light green solution; the solution turns blue litmus paper red | C is a soluble salt; the solution is acidic |
| (b) First portion + NaOH(aq) dropwise then in excess, then heat and keep for a few minutes | A green gelatinous precipitate forms which is insoluble in excess NaOH; on heating a colourless, pungent, choking gas is evolved which turns damp red litmus paper blue and gives dense white fumes with conc. HCl; the precipitate slowly turns brown at the upper part of the tube | Fe2+ present (green Fe(OH)2); NH4+ present (ammonia gas from the ammonium ion); the Fe2+ is oxidized to Fe3+ in air, giving the brown colour |
| (c) Second portion + a few drops of BaCl2(aq), then excess dilute HCl | A white precipitate forms which does not dissolve in excess dilute HCl | SO42- present (white BaSO4, insoluble in acid) |
Conclusion: C contains Fe2+, NH4+ and SO42- ions, so C is the double salt ammonium iron(II) tetraoxosulphate(VI), \((NH_4)_2SO_4 \cdot FeSO_4 \cdot 6H_2O\).
