(a) If you were provided with anhydrous Na\(_2\)CO\(_3\), spatula and stirrer;
(i) list three other materials you would require to prepare a standard solution of Na\(_2\)CO\(_3\)
(ii) state what you would observe on adding diluted H\(_2\)SO\(_4\) to a portion of the Na\(_2\)CO\(_3\)
(b)(i) Describe briefly one chemical test you would perform to distinguish between zinc ions and aluminium ions in solution.
(ii) Mention one laboratory reagent you would use to;
I. produce ammonia from (NH\(_4\))\(_2\)SO\(_4\)
II. differentiate between precipitates of AgCl and Agl
lll. dehydrate ethanol
(c) Give the reason for each of the following laboratory practices
(i) Aqueous solutions of FeSO\(_4\) are freshly prepared when required for use.
(ii) The first jar of hydrogen collected during its preparation is discarded
(a) Standard solution of Na2CO3
(i) Three other materials required: a chemical (weighing) balance, a volumetric (standard) flask, and a wash bottle of distilled water (a funnel and a beaker may also be used).
(ii) On adding dilute H2SO4 to the Na2CO3: brisk effervescence occurs and a colourless, odourless gas (CO2) is evolved which turns lime water milky.
\[ Na_2CO_3 + H_2SO_4 \to Na_2SO_4 + H_2O + CO_2 \]
(b)(i) Distinguishing Zn2+ from Al3+
Add aqueous ammonia dropwise then in excess to each solution. Both give a white gelatinous precipitate; with Zn2+ the precipitate dissolves in excess ammonia (forming a colourless complex), whereas with Al3+ the precipitate is insoluble in excess ammonia. This distinguishes them.
(ii) Laboratory reagents:
- I. Produce NH3 from (NH4)2SO4: warm with sodium hydroxide (or calcium hydroxide).
- II. Differentiate AgCl from AgI: aqueous ammonia - AgCl dissolves, AgI does not.
- III. Dehydrate ethanol: concentrated sulphuric acid (or hot aluminium oxide).
(c) Reasons
(i) FeSO4 solutions are freshly prepared because the Fe2+ ions are readily oxidised by air (oxygen) to Fe3+, so a fresh solution is needed to keep the iron as pure Fe2+.
(ii) The first jar of hydrogen is discarded because it is mixed with the air already in the apparatus, forming an explosive mixture; discarding it ensures the hydrogen collected afterwards is pure and safe.
(a) Standard solution of Na2CO3
(i) Three other materials required: a chemical (weighing) balance, a volumetric (standard) flask, and a wash bottle of distilled water (a funnel and a beaker may also be used).
(ii) On adding dilute H2SO4 to the Na2CO3: brisk effervescence occurs and a colourless, odourless gas (CO2) is evolved which turns lime water milky.
\[ Na_2CO_3 + H_2SO_4 \to Na_2SO_4 + H_2O + CO_2 \]
(b)(i) Distinguishing Zn2+ from Al3+
Add aqueous ammonia dropwise then in excess to each solution. Both give a white gelatinous precipitate; with Zn2+ the precipitate dissolves in excess ammonia (forming a colourless complex), whereas with Al3+ the precipitate is insoluble in excess ammonia. This distinguishes them.
(ii) Laboratory reagents:
- I. Produce NH3 from (NH4)2SO4: warm with sodium hydroxide (or calcium hydroxide).
- II. Differentiate AgCl from AgI: aqueous ammonia - AgCl dissolves, AgI does not.
- III. Dehydrate ethanol: concentrated sulphuric acid (or hot aluminium oxide).
(c) Reasons
(i) FeSO4 solutions are freshly prepared because the Fe2+ ions are readily oxidised by air (oxygen) to Fe3+, so a fresh solution is needed to keep the iron as pure Fe2+.
(ii) The first jar of hydrogen is discarded because it is mixed with the air already in the apparatus, forming an explosive mixture; discarding it ensures the hydrogen collected afterwards is pure and safe.
