Preparation of Salts (Cambridge (CIE) O Level Chemistry): Exam Questions

Soluble salts can be made by the neutralisation of an acid by a base. Insoluble salts can be made by precipitation.

The following is a brief description of the preparation of the soluble salt, nickel(II) chloride-6-water, from the insoluble base nickel(II) carbonate. Nickel(II) carbonate is added in small amounts to hot dilute hydrochloric acid until it is in excess. The mixture is filtered. The filtrate is partially evaporated and then allowed to cool until crystals of nickel(II) chloride-6-water form.

i) Why is it necessary to use excess carbonate?

[1]

ii) Explain why it is necessary to filter.

[1]

iii) Why partially evaporate rather than evaporate to dryness?

[1]

iv) What additional steps are needed to obtain dry crystals?

[2]

Potassium chloride can be made from hydrochloric acid and potassium carbonate.

i) Why must a different experimental method be used for this preparation?

[1]

ii) Give a description of the different method used for this salt preparation.

[4]

Insoluble salts are made by precipitation. An equation for the preparation of barium sulfate is given below.

BaCl₂ (aq) + MgSO₄ (aq) → BaSO₄ (s) + MgCl₂ (aq)

This reaction can be used to find x in the formula for hydrated magnesium sulfate MgSO₄.xH₂O. 

A known mass of hydrated magnesium sulfate, MgSO₄.xH₂O, was dissolved in water. Excess aqueous barium chloride was added. The precipitate of barium sulfate was filtered, washed and dried. Finally it was weighed.

Mass of hydrated magnesium sulfate = 1.476 g Mass of barium sulfate formed = 1.398 g The mass of one mole of BaSO4 = 233 g

The number of moles of BaSO₄ formed = ...............

[1]

The number of moles of MgSO₄.xH₂O = ...............

[1]

The mass of one mole of MgSO₄.xH₂O = ...............g

[1]

The mass of one mole of MgSO₄= 120 g

The mass of xH₂O in one mole of MgSO₄.xH₂O = ...............

[1]

x = ...............

[1]

Three common methods of preparing salts are shown below.

For each of the following salt preparations, choose a method, A, B or C. Name any additional reagent which is needed and complete the equation.

The soluble salt, nickel chloride, from the insoluble compound nickel carbonate

method ..........................................................................................................

reagent ..........................................................................................................

word equation ..........................................................................................................

Separate: Chemistry and Extended Only

The insoluble salt, lead(II) bromide, from aqueous lead(II) nitrate

method .......................................................................................................................................

reagent ......................................................................................................................................

ionic equation ........ + ........ → PbBr₂

The soluble salt, lithium sulfate, from the soluble base lithium

hydroxide method .......................................................................................................................................

reagent ......................................................................................................................................

equation .....................................................................................................................................

Extended Only

Three ways of making salts are

• titration using a soluble base or carbonate

• neutralisation using an insoluble base or carbonate

• precipitation

Complete the following table of salt preparations.

i) Write an ionic equation with state symbols for the preparation of silver(I) chloride.

[2]

ii) Complete the following equation.

ZnCO₃ + H₂SO₄ → ............... + ............... + ...............

[2]

Chlorine reacts with phosphorus to form phosphorus trichloride.

Draw a diagram showing the arrangement of the outer electrons in one molecule of the covalent compound, phosphorus trichloride.

Use x to represent an electron from a phosphorus atom.
Use o to represent an electron from a chlorine atom.

Extended Only

Phosphorus trichloride reacts with water to form two acids.

i) Balance the equation for this reaction.

PCl₃ + .......H₂O → .......HCl + H₃PO₃

[1]

ii) Describe how you could show that phosphorus acid, H₃PO₃, is a weaker acid than hydrochloric acid.

[3]

iii) Two salts of phosphorus acid are its sodium salt, which is soluble in water, and its calcium salt which is insoluble in water. Suggest a method of preparation for each of these salts from aqueous phosphorus acid. Specify any other reagent needed and briefly outline the method.

Sodium salt

[2]

   Calcium salt

[2]

Oxides are classified as acidic, basic, neutral and amphoteric.

Complete the table.

Extended Only

i) Explain the term amphoteric.

[1]

ii) Name two reagents that are needed to show that an oxide is amphoteric.

[2]

There are three methods of preparing salts.

• Method A – use a burette and an indicator.

• Method B – mix two solutions and obtain the salt by precipitation.

• Method C – add an excess of base or a metal to a dilute acid and remove the excess by filtration.

For each of the following salt preparations, choose one of the methods A, B or C, name any additional reagent needed and then write or complete the equation.

 i) The soluble salt, zinc sulfate, from the insoluble base, zinc oxide

method ..................................................

 reagent ..................................................

 word equation ..................................................

[3]

ii) The soluble salt, potassium chloride, from the soluble base, potassium hydroxide

method ..................................................

reagent ..................................................

equation ..................... + .....................  → KCl + H₂O

[3]

iii)

The insoluble salt, lead(II) iodide, from the soluble salt, lead(II) nitrate

method ..................................................

 reagent ..................................................

 equation Pb²⁺ + ..................... → ..................... 

[4]

Which one of these salts is soluble in water?
Tick one box.

Carbon dioxide is a gas.

Which one of these processes does not produce carbon dioxide?

Tick one box.

Concentrated sulfuric acid is a dehydrating agent which can chemically remove water from substances.

Both hydrated copper(II) sulfate crystals and sucrose (a sugar), C₁₂H₂₂O₁₁, can be completely dehydrated by concentrated sulfuric acid.

Name the solid product formed in each case.

hydrated copper(II) sulfate crystals ________________

sucrose ________________

This question is about copper and copper compounds.

Describe how you could prepare a pure sample of crystals of hydrated copper(II) sulfate using dilute sulfuric acid and an excess of copper(II) oxide.

Silver chloride, AgCl, is insoluble. It can be made by a precipitation reaction between aqueous barium chloride and a suitable aqueous silver salt.

What is meant by the term precipitate?

Separate: Chemistry and Extended Only

Name a suitable silver salt to use to prepare silver chloride.

Complete the chemical equation to show the formation of insoluble silver chloride from aqueous barium chloride and the silver salt you have named.

name of a suitable silver salt ...........................................

BaCl₂ + ..................... ..................... + .....................

Hydriodic acid, HI (aq), is a strong acid. Its salts are iodides.

It has the reactions of a typical strong acid. Complete the following equations.

i) ............Li + ............HI → ........................ + ...................

[1]

ii) Zinc carbonate + hydriodic acid → .................... + .................... + ....................

[1]

iii) MgO + ............HI → ................... + ...................

[1]

Two of the reactions in (a) are acid / base and one is redox. Which one is redox? Explain your choice.

Describe how you could distinguish between hydriodic, HI (aq), and hydrobromic, HBr (aq) acids, by bubbling chlorine through these two acids.

Result with hydriodic acid ..................................................................................................

Result with hydrobromic acid .......................................................................................

20.0 cm³ of aqueous sodium hydroxide, 2.00 mol / dm³, was placed in a beaker.

The temperature of the alkali was measured and 1.0 cm³ portions of hydriodic acid were added.

After each addition, the temperature of the mixture was measured. Typical results are shown on the graph.

NaOH (aq) + HI (aq) → NaI (aq) + H₂O (l)

i) Explain why the temperature increases rapidly at first and then stops increasing.

[2]

ii) Suggest why the temperature drops after the addition of 18.0 cm³ of acid.

[1]

iii) In another experiment, it was shown that 15.0 cm³ of the acid neutralised 20.0 cm³ of aqueous sodium hydroxide, 1.00 mol / dm³. Calculate the concentration of the acid.

[2]

The hydroxides of the Group I metals are soluble in water. Most other metal hydroxides are insoluble in water.

Crystals of lithium chloride can be prepared from lithium hydroxide by titration.

25.0cm³ of aqueous lithium hydroxide is pipetted into the conical flask.

A few drops of an indicator are added. Dilute hydrochloric acid is added slowly to the alkali until the indicator just changes colour. The volume of acid needed to neutralise the lithium hydroxide is noted.

A neutral solution of lithium chloride, which still contains the indicator, is left. Describe how you could obtain a neutral solution of lithium chloride which does not contain an indicator.

The concentration of the hydrochloric acid was 2.20 mol / dm³. The volume of acid needed to neutralise the 25.0 cm³ of lithium hydroxide was 20.0 cm³. Calculate the concentration of the aqueous lithium hydroxide.

LiOH + HCl → LiCl + H₂O

Lithium chloride forms three hydrates. They are LiCl.H₂O, LiCl.2H₂O and LiCl.3H₂O.  

Which one of these three hydrates contains 45.9% of water? Show how you arrived at your answer.

Soluble salts can be made using a base and an acid.

Complete this method of preparing dry crystals of the soluble salt cobalt(II) chloride-6-water from the insoluble base cobalt(II) carbonate.

Step 1

Add an excess of cobalt(II) carbonate to hot dilute hydrochloric acid.

Step 2

............................................................................................................

Step 3

............................................................................................................

Step 4

............................................................................................................

[4]

5.95g of cobalt(II) carbonate were added to 40 cm³ of hydrochloric acid, concentration 2.0 mol / dm³.  

i) Calculate the maximum yield of cobalt(II) chloride-6-water and show that the cobalt(II)carbonate was in excess.

CoCO₃ + 2HCl → CoCl₂ + CO₂ + H₂O

CoCl₂ + 6H₂O → CoCl₂.6H₂O

Maximum yield:

number of moles of HCl used = ..............................

number of moles of CoCl₂ formed = ..............................

number of moles of CoCl₂.6H₂O formed = ..............................

mass of one mole of CoCl₂.6H₂O = 238 g

maximum yield of CoCl₂.6H₂O = .............................. g

To show that cobalt(II) carbonate is in excess:

number of moles of HCl used = .............................. (use your value from above)

mass of one mole of CoCO₃ = 119g

number of moles of CoCO₃ in 5.95g of cobalt(II) carbonate = ..............................

[5]

ii) Explain how these calculations show that cobalt(II) carbonate is in excess.

[1] 

Silver(I) chromate(VI) is an insoluble salt. It is prepared by precipitation. 20 cm³ of aqueous silver(I) nitrate, concentration 0.2 mol /dm³, was mixed with 20 cm³ of aqueous potassium chromate(VI), concentration 0.1 mol /dm³. After stirring, the mixture was filtered. The precipitate was washed several times with distilled water. The precipitate was then left in a warm oven for several hours.

2AgNO₃ (aq) + K₂CrO₄ (aq) → Ag₂CrO₄ (s) + 2KNO₃ (aq)

What difficulty arises if the name of a compound of a transition element does not include its oxidation state, for example iron oxide?

Extended Only

These questions refer to the preparation of the salt.

i) Why is it necessary to filter the mixture after mixing and stirring?

[1]

ii) What is the purpose of washing the precipitate?

[1]

iii) Why leave the precipitate in a warm oven?

[1]

i) Explain why the concentrations of silver(I) nitrate and potassium chromate(VI) are different.

[1]

ii) What mass of silver(I) nitrate is needed to prepare 100 cm³ of silver(I) nitrate solution, concentration 0.2 mol / dm³?

The mass of one mole of AgNO₃ is 170 g.

[2]

iii) What is the maximum mass of silver(I) chromate(VI) which could be obtained from 20 cm³ of aqueous silver(I) nitrate, concentration 0.2 mol / dm³?

Number of moles of AgNO₃ used = .....................

[1]

Number of moles of Ag₂CrO₄ formed = .....................

[1]

Mass of one mole of Ag₂CrO₄ = 332 g

Mass of Ag₂CrO₄ formed = ..................... g

[1]

Extended Only

Insoluble salts are made by precipitation.

A preparation of the insoluble salt calcium fluoride is described below.

To 15 cm³ of aqueous calcium chloride, 30 cm³ of aqueous sodium fluoride is added. The concentration of both solutions is 1.00 mol / dm³. The mixture is filtered and the precipitate washed with distilled water. Finally, the precipitate is heated in an oven.

i) Complete the equation.

Ca²⁺ + ..........F^- →...............

[2]

ii) Why is the volume of sodium fluoride solution double that of the calcium chloride solution?

[1]

iii) Why is the mixture washed with distilled water?

[1]

iv) Why is the solid heated?

[1]

The formulae of insoluble compounds can be found by precipitation reactions. To 12.0 cm³ of an aqueous solution of the nitrate of metal T was added 2.0 cm³ of aqueous sodium phosphate, Na₃PO₄. The concentration of both solutions was 1.00 mol/ dm³.

When the precipitate had settled, its height was measured.

The experiment was repeated using different volumes of the phosphate solution. The results are shown on the following graph.

What is the formula of the phosphate of metal T? Give your reasoning.