Prior knowledge required in this topic – Solubility Rules

1. Prior knowledge required in this topic – Solubility Rules • All chlorides are soluble except those of lead and silver. • All sulfates are soluble except those of barium, calcium and lead. • All nitrates are soluble. • All compounds of Gp I (Na/K) and ammonium are soluble. • All hydroxides and carbonates are insoluble except those of Gp I (Na/K) and ammonium • All common lead and silver salts are insoluble except those of their nitrates. (Silver sulfate is partially soluble)

2. Answers to Preparation of Salts • MCQ • C • D • D • A • D • D 7. D 8. A 9. B 10. B 11. A 12. C

3. Section B, Question 1 • sodium nitrate • (Recall titration method is used to prepare Gp I and ammonium salt) • b) calcium carbonate and barium sulfate • c) zinc chloride and iron (II) sulfate • (copper (II) sulfate not acceptable as copper is not reactive enough to react with dilute acid) • d) calcium carbonate • e) barium sulfate and calcium carbonate • (Recall that precipitation method is used to prepare insoluble salts)

4. Important concept which is still lacking in students • To prepare soluble salt, acid must be one of the reagents. The 2nd reagent must be something that can react with acid, i.e, metal/base/carbonate. • acid + metal/insoluble base/insoluble carbonate • (Excess insoluble reactant added, acid warmed, filter to remove excess reactant, heat to obtain a saturated solution, crystallisation, drying) • acid + soluble base/carbonate • (method is titration!) 2. For insoluble salt, method is precipitation. • Cannot use acid + insoluble base/carbonate because the insoluble salt forms a coating over solid reactant.

5. Question 2 • magnesium/magnesium oxide/ • magnesium hydroxide/magnesium carbonate and dilute hydrochloric acid • aqueous ammonia (an alkali) and dilute sulfuric acid • (ammonium hydroxide NOT accepted) • copper (II) oxide/copper (II) hydroxide/copper (II) carbonate and dilute sulfuric acid • (Copper not accepted, because it is not reactive enough to react with dilute acid) • d) dilute hydrochloric acid/aqueous sodium chloride/ • aqueous potassium chloride and aqueous lead (II) nitrate • (For precipitation, reagents must be aqueous, state clearly!)

6. Question 2 • aqueous sodium hydroxide/aqueous sodium carbonate and dilute nitric acid • Aqueous zinc nitrate and • aqueous sodium carbonate/potassium carbonate • (Carbonic acid not a good answer as it is a weak acid and does not give enough carbonate ions needed for precipitation) • Note: • For acid, important to state it is dilute. • For preparation of insoluble salts by precipitation, important to state that the reagents are in aqueous form.

7. Question 3 • Potassium hydroxide/potassium carbonate • (Potassium oxide not accepted. The method used here is titration and reagents should be in solution form. Potassium oxide is a solid, when dissolved it gives KOH, so essentially it is KOH that is titrated with sulfuric acid. K2O + H2O  2KOH ) • b) 2KOH (aq) + H2SO4 (aq)  K2SO4 (aq) + 2H2O (l) • or K2CO3 (aq) + H2SO4 (aq)  K2SO4 (aq) + • H2O (l) + CO2 (g) • Method: titration • Type of reaction: neutralisation

8. *Question 3, part ( d ) - titration Pipette 20.0 cm3 of potassium hydroxide into a conical flask. (Cannot use beaker here!) Add two drops of methyl orange , the solution should turn yellow. (Cannot use Universal indicator for titration, no distinct end pt!) Titrate the solution in the conical flask with dilute sulfuric acid from a burette until end point is reached, when the solution turns from yellow to orange. Record the volume of acid added. Can also write “Titrate 20.0 cm3 of KOH with dilute sulfuric acid using methyl orange as indicator.” This means KOH is in flask and sulfuric acid is in burette.

9. Question 3, part (c) continued Repeat the experiment with 20.0 cm3 of potassium hydroxide and the volume of dilute sulfuric acid obtained from titration, but without the indicator. Stir the mixture and heat to evaporate some water to obtain a saturated solution. Cool the hot and saturated solution so that crystallisation occurs and crystals are formed. Filter to obtain crystals as residue. Dry the crystals by squeezing them between sheets of filter papers.

10. 3 e) Potassium sulfate as well as the reagents used to prepare it (aqueous potassium hydroxide and dilute sulfuric acid) are all soluble. Exact quantities of reagents must be added to prevent contamination of salt/to obtain a pure salt. Hence the titration method which involves the use of an indicator must be used, so that the exact quantities of reagents can be determined.

11. Question 4 • b) No. Lead (II) sulfate formed is insoluble and would form a coating over lead (II) carbonate, preventing the dilute sulfuric acid from coming into contact with lead (II) carbonate and stopping the reaction. • i) aqueous lead (II) nitrate • and dilute sulfuric acid/aqueous sodium sulfate • ii) Filter the mixture to obtain lead (II) sulfate as the residue. • Wash the residue with copious amounts of deionised or distilled water to remove any soluble impurities. • Dry the salt between sheets of filter paper. • (Must filter before washing !! No heating required here, salt is insoluble)

12. Question 5 • Carbon dioxide • To ensure that the magnesium carbonate is added in excess so that all the dilute sulfuric acid is reacted. • If the filtrate was evaporated to dryness, the hydrated magnesium sulfate wouldlose its water of crystallisationand becomeanhydrous. • Effervescence has stopped. Some powder remained at the bottom of the beaker. • (Question asks for evidence, read passage for these lines “ effervescence stopped and some powder remained at the bottom…”) • Magnesium oxide/magnesium metal/ • magnesium hydroxide (need to choose a base or metal here)