A LevelChemistryCIETopic Questions2. Atoms, Molecules & Stoichiometry2.3 Formulas

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First teaching 2023

First exams 2025

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Formulas (CIE A Level Chemistry)

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1a3 marks

Complete Table 1.1.

Table 1.1
 
Ion Formula
Nitrate NO3–
  CO32–
Sulfate  
  OH–
  NH4+
Zinc(II)  
  Ag+
  HCO3–
Phosphate  

 

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1b3 marks

Complete Table 1.2.

Table 1.2
 
Ionic compound Formula
Sodium chloride NaCl
Potassium oxide  
  Al2(CO3)3
Copper(II) nitrate  

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1c3 marks

Aqueous silver nitrate should not be acidified with hydrochloric acid as it forms a white precipitate of silver chloride, AgCl, and nitric acid.

i)
Write the chemical equation for the reaction of silver nitrate with hydrochloric acid. Your answer should include state symbols.
[2]
ii)
Silver ions from the silver nitrate and hydrogen ions from the hydrochloric acid are responsible for the formation of silver chloride.
Ag+ + Cl– → AgCl

Suggest why the nitrate ions and hydrogens do not appear in this ionic equation.

[1]

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1d3 marks

Aqueous barium chloride should not be acidified with sulfuric acid as it forms a white precipitate of barium sulfate and hydrochloric acid.

i)
Write the chemical equation for the reaction of barium chloride with sulfuric acid. Your answer should include state symbols.
[2]
ii)
Write the ionic equation for the formation of barium sulfate. 
[1]

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2a2 marks
Explain the meaning of the terms empirical formula and molecular formula.
 

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2b4 marks

Three organic compounds are shown in Fig. 5.1.

1-2-5b-e-ch2o-empirical-formula-chemicals
Fig. 5.1
 
i)
Give the molecular formula of each compound.
 
Acetic acid ..............................
 
Glucose ..............................
 
Lactic acid ..............................
[3]
 
ii)
Give the empirical formula of all three compounds.
 
[1]

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2c
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An unknown hydrocarbon contains 85.7% carbon. The relative molecular mass of the unknown hydrocarbon is 56.0. 

i)
Calculate the empirical formula of the unknown hydrocarbon. Show your working.
[3]
 
ii)
Determine the molecular formula of the unknown hydrocarbon. Show your working.
[2]

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2d1 mark

Explain the difference between an anydrous salt and a hydrated salt.

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2e
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Gypsum is a salt that can contain water of crystallisation. In its anhydrous form, the chemical formula of gypsum is CaSO4. 1.80 g of water is added to 6.81 g of gypsum to form the hydrated salt, CaSO4•xH2O. 

Calculate the value of x in CaSO4•xH2O. Show your working.

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1a
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Magnesium carbonate is a common antacid that is used to react with and neutralise excess stomach acid, HCl. 

MgCO3 (s) + 2HCl (aq) rightwards arrow MgCl2 (aq) + CO2 (g) + H2O (l)

The total mass of the reactants is 9.91 g.

Calculate the volume of carbon dioxide that is produced at room temperature and pressure.


Volume of CO2 = .................. dm3

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1b
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Another magnesium salt, magnesium sulfate, can exist in hydrated or anhydrous forms.

6.31 g of a sample of magnesium sulphate, MgSO4.xH2O, was strongly heated until a mass of 3.61 g of solid remained with no further change in mass recorded.

Calculate the number of moles of water of crystallisation, x, in the magnesium sulphate sample.

moles of water = ..................... mol

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1c
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Another sample of hydrated magnesium sulphate, MgSO4.xH2O, is found to contain 51.1% water by mass.

Calculate the number of moles of water of crystallisation, x, in this sample.


moles of water = ..................... mol

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1d2 marks

Describe the effect on the value of x obtained if the salt was not heated strongly to a constant mass.

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2a
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The coca-cola company states that a 12 oz can of coke contains 34 mg of caffeine, Mr = 194.19 g mol-1.

Calculate the number of moles of caffeine in a can of coke.


moles of caffeine = .................... mol

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2b
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Caffeine is made up of 49.48%  carbon, 5.20% hydrogen, 28.85% nitrogen and the rest is oxygen.

Calculate the empirical formula of caffeine.

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2c
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Calculate the molecular formula of caffeine using your answer from part (b).

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3a5 marks

Apatite is a general term used for a family of various calcium phosphate compounds commonly found in rocks and minerals, bones and teeth.

i)
State the formula of calcium phosphate.
[1]
 
ii)
Hydroxylapatite, Ca5(PO4)3OH, is one member of the apatite family and is a major component of bone.
Give the oxidation number of calcium in hydroxyapatite. Explain your answer.
[3]
 
iii)
Another member of the apatite family is chlorapatite.
Suggest the formula of chlorapatite.
[1]

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3b2 marks

Fluorapatite, Ca5(PO4)3F, reacts with sulfuric acid to form hydrogen phosphate, calcium sulfate and hydrogen fluoride.

Write the balanced chemical equation for this reaction.

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3c
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Fluorapatite, Ca5(PO4)3F, can also react with sulfuric acid to form calcium dihydrogenphosphate, Ca(H2PO4)2, calcium sulfate and hydrogen fluoride.

2Ca5(PO4)3F + 7H2SO4 → 3Ca(H2PO4)2 + 7CaSO4 + 2HF

Calculate the mass of calcium dihydrogenphosphate formed when 50.2 g of Ca5(PO4)3F reacts with 87.9 g H2SO4.


mass of Ca(H2PO4)2 = ............................ g 

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1a
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Cobalt(II) sulfate is used in storage batteries, electroplating baths and as an additive to soils and animal feeds.

A sample of hydrated cobalt sulfate, CoSO4•xH2O, has a relative formula mass of 173.0.

Calculate a value for 'x'.

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1b
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A student completed an experiment to determine the value of ‘x’ in a hydrated salt.

They heated a solid sample of the salt, CoSO4.xH2O, in a crucible for 60 seconds and recorded the following set of results.

Table 2.1
 

Mass of empty crucible (g)

29.22

Mass of crucible and solid before heating (g)

32.67

Mass of crucible and solid after heating (g)

32.02

 

Use the data obtained by the student in Table 2.1 to calculate a value for ‘x’.

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1c3 marks

The actual value for x in CoSO4.xH2O is 6.

i)
Suggest one reason for the difference between your calculated value in part (b) and the actual value.
[1]
 
ii)
Explain how you could improve the experiment the student did, using the same equipment, to achieve a value closer to the correct value for x.
[2]

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1d1 mark

Cobalt(II) sulfate undergoes a displacement reaction with magnesium because magnesium is a more reactive metal than cobalt.

Write an ionic equation for this reaction. State symbols are not required.

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2a2 marks

Chevreul's salt or copper(I, II) sulfite dihydrate, Cu3(SO3)2•2H2O, is an unusual hydrated salt as it contains copper in both of its common oxidation states.

The first step in the formation of Chevreul's salt is the reaction of aqueous copper sulfate with a solution of potassium metabisulfite, K2S2O5.

Balance the equation for the reaction to form the anhydrous Chevreul's salt.

__CuSO4 + __K2S2O5 → Cu3(SO3)2 + 4K2SO4 + __SO2 

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2b
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The chemical formula of Chevreul's salt is sometimes written as Cu2SO3•CuSO3•2H2O to show that it contains copper(I) sulfite and copper(II) sulfite.

Calculate the percentage by mass of copper(I) ions in copper(I) sulfite

Show your working.

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2c2 marks

Chevreul's salt can react with ammonia solution to form a deep blue solution of the tetraamminecopper(II) complex, [Cu(NH3)4]2+ (aq). It can also react with hydrochloric acid to form a white solid of copper(I) chloride, water and sulfur dioxide.

Write an ionic equation for the formation of copper(I) chloride from Chevreul's salt. State symbols are not required.

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2d
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When Chevreul's salt is heated in an inert atmosphere, it is stable to around 200 oC. Above this temperature, a variety of compounds can be produced although the final product of sustained heating at high temperatures is copper(II) oxide.

During the heating process, a sample of the mixture is taken. After separation, the most common compound was analysed and found to contain 39.8% copper, 20.1% sulfur and the rest oxygen.

Calculate the empirical formula of the most common compound. Show your working.

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3a3 marks

A synthetic carbohydrate was isotopically labelled with carbon-13.

Analysis of a 12 g sample of the carbohydrate found it to contain 6.93 g carbon, 0.80 g hydrogen and the remainder was oxygen.

Determine the empirical formulae of the carbohydrate.

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3b1 mark

The fermentation of glucose, C6H12O6, is one method scientists can use to produce ethanol.  

Write a balanced symbol equation for this reaction. 

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3c3 marks

During the fermentation process in the reaction in part (b), 312 g of ethanol was produced from 1.5 kg of glucose. 

Calculate the percentage yield of this reaction to 1 decimal place. If you were unable to produce an equation for part (b), assume that 1 mole of glucose would produce 4 moles of ethanol for this question (this is not the correct molar ratio). 

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