Use a copy of the Periodic Table where you need one.
Which pair of reagents would be most suitable to safely prepare a pure sample of the salt in the laboratory?
| reagents | salt | |
|---|---|---|
| A | potassium and dilute hydrochloric acid | potassium chloride |
| B | calcium oxide and dilute sulfuric acid | calcium sulfate |
| C | copper and dilute sulfuric acid | copper(II) sulfate |
| D | silver nitrate and dilute hydrochloric acid | silver chloride |
Source: 2026 Chung Cheng Prelim P1 Q16
Which pair of chemicals is most suitable to prepare a pure sample of copper(II) nitrate?
| chemical 1 | chemical 2 | |
|---|---|---|
| A | copper | dilute nitric acid |
| B | copper | aqueous potassium nitrate |
| C | copper(II) carbonate | dilute nitric acid |
| D | copper(II) carbonate | aqueous potassium nitrate |
Source: Broadrick Prelim P1 Q22
Which set of reactants is most appropriate to prepare a pure sample of copper(II) chloride crystals?
Source: 2024 AMKSS Prelim P1 Q12
Dilute sulfuric acid reacts with aqueous sodium hydroxide: H2SO4 + 2NaOH → Na2SO4 + 2H2O. Sodium sulfate solution is obtained at the end of the reaction.
Source: 2023 GESS Sec 3 End-of-Year P2 B11(c) and (d)
Magnesium reacts with dilute nitric acid to form magnesium nitrate and hydrogen.
Source: 2023 GESS Sec 3 End-of-Year P2 B12(d)
Salts that are insoluble in water are prepared by precipitation.
Source: 2024 TKGS Prelim P2 Q3(a)
Barium forms several compounds. The table shows some of their properties.
| compound | solubility in water | uses |
|---|---|---|
| barium carbonate | insoluble | rat poison (toxic due to barium ions) |
| barium chloride | soluble | heat treatment of steel tools |
| barium hydroxide | soluble | used in acid-base titrations |
| barium sulfate | insoluble | contrast agent for X-rays |
Source: 2026 NAS Prelim P2 Q11
Dsilver nitrate and dilute hydrochloric acid[1]
Silver chloride is insoluble, so it is precipitated from two solutions. Potassium reacts violently with acid, copper does not react with dilute acids, and insoluble calcium sulfate coats the calcium oxide.
Ccopper(II) carbonate and dilute nitric acid[1]
Copper(II) nitrate is soluble, so an excess of insoluble copper(II) carbonate is added to the acid and filtered off. Copper does not react with dilute acid, and potassium nitrate is not an acid.
Bcopper(II) carbonate and hydrochloric acid[1]
Copper(II) chloride is soluble, so an excess of insoluble copper(II) carbonate is reacted with the acid and filtered off. Copper does not react with dilute acid, and the soluble salts in D give no precipitate to separate.
The colourless sodium sulfate solution turns green. [1]
The concentrations of H+ and OH− ions are equal. [1]
Pipette 25.0 cm3 of aqueous sodium hydroxide into a conical flask, add 1 to 2 drops of methyl orange, and fill a burette with dilute sulfuric acid. [1]
Add the acid slowly while swirling, until 1 drop turns the indicator from yellow to orange: the end-point. [1]
Record the volume of acid needed, (V2 − V1) cm3. [1]
Repeat with 25.0 cm3 of sodium hydroxide and (V2 − V1) cm3 of acid, without the indicator. [1]
Heat until saturated, cool to crystallise, filter, wash with a little cold distilled water and dry the crystals with filter paper. [1]
With constant stirring, add excess magnesium to dilute nitric acid until no more effervescence is observed. [1]
Filter to remove the excess magnesium as the residue, and collect the filtrate, magnesium nitrate solution. [1]
Heat the filtrate until it is saturated, then allow it to cool and crystallise. [1]
Filter to collect the crystals, wash them with a little cold distilled water and dry them between sheets of filter paper. [1]
Lead(II) nitrate, Pb(NO3)2, and sodium iodide, NaI (any two soluble salts containing Pb2+ and I− ions). [1]
Add excess aqueous lead(II) nitrate to aqueous sodium iodide and stir until no more precipitate forms. Filter the mixture to collect the lead(II) iodide precipitate. [1]
Wash the precipitate with distilled water to remove impurities, and dry it between a few sheets of filter paper. [1]
Pb2+(aq) + 2I−(aq) → PbI2(s) [1]
Filter the mixture to remove the excess barium carbonate and obtain barium chloride solution as the filtrate. [1]
Heat the filtrate to form a saturated solution, then cool it to allow crystals to form. [1]
Filter to obtain the crystals, wash them with a little cold distilled water and dry them between sheets of filter paper. [1]
Barium carbonate reacts with the acid in the stomach to form (soluble) barium ions, which are toxic. [1]
Mr of BaCl2 = 137 + 2(35.5) = 208 and Mr of BaCl2•2H2O = 208 + 2(18) = 244 [1]
Mass of BaCl2 = 208244 × 4.88 = 4.16 g [1]
Ba(OH)2(aq) + H2SO4(aq) → BaSO4(s) + 2H2O(l) [1]
A white precipitate is observed. [1]