Salt Preparation Questions on Solubility
2019 O Level, Open-ended Question A3
Some gold ores are mixed with impurities. These impurities include compounds of lead.
The lead compounds can be removed from the gold by reacting the ore with dilute nitric acid.
Dilute hydrochloric acid or dilute sulfuric acid cannot be used to remove compounds of lead.
Explain why dilute nitric acid can be used to remove compounds of leads but these acids cannot be used.
The chloride ions from hydrochloric acid and the sulfate ions from sulfuric acid would combine with lead(II) ions in the impure ore to form insoluble lead(II) chloride and lead(II) sulfate. The insoluble salt will coat the unreacted compounds of leads, preventing further reaction to fully react away the impurities.
However, all nitrate salts are soluble. Nitrate ions from nitric acid will remain in solution, and no solid coating will be formed.
Marker’s notes: As lead is a metal, it mainly forms ionic compounds like lead(II) oxide. Therefore, the element lead would exist as lead(II) ions in the impure ore.
Salt Preparation Questions on Reagents and Methods
2018 O Level, MCQ 23
Potassium chloride is a salt that can be prepared in the laboratory.
Which reagents are the most suitable for the safe preparation of a pure sample of potassium chloride?
A. aqueous potassium hydroxide and dilute hydrochloric acid
B. aqueous potassium nitrate and aqueous sodium chloride
C. potassium and chlorine
D. potassium and dilute hydrochloric acid
In option A, potassium hydroxide and hydrochloric acid undergo neutralisation to form potassium chloride and water. As the reactants — aqueous potassium hydroxide, dilute hydrochloric acid — and potassium chloride are all soluble, the reactants must be added in exact amount via titration. After titration, the resulting potassium chloride solution will be heated to derive potassium chloride by evaporation to dryness.
Potassium is a reactive Group I metal. In fact, being lower down Group I, it is more reactive than sodium. Therefore, it cannot be used directly to react with either chlorine or dilute hydrochloric acid, lest an explosion occurs. Therefore, options C and D are out.
In option B, the by-product of soluble sodium nitrate will contaminate the solution of potassium chloride formed. As there is no easy way to separate soluble sodium nitrate from soluble potassium chloride, option B is unsuitable.
2018 O Level, Open-ended A3
What other reagent is needed to form copper(II) chloride salt from aqueous hydrochloric acid by the addition of excess solid to acid? Write the formula of the reagent, including state symbol.
CuO(s) or CuCO3(s)
Marker’s notes: copper cannot be used as it is an unreactive metal that does not react with acids
Aqueous potassium hydroxide and aqueous nitric acid are used to form a salt. State the formula of the salt, including state symbol, and the suitable method of salt preparation.
The salt formed is potassium nitrate. The method is titration, whereby exact amounts of reactants are added to prevent any excess from contaminating the resulting solution of potassium nitrate.
State the formulae of reagents needed to prepare lead(II) sulfate. Outline the two key steps of the suitable method to prepare it.
The reagents are Pb(NO3)2(aq) and Na2SO4(aq). The method involves precipitation, followed by filtration to collect the lead(II) sulfate precipitate as the residue.