Formulae (Cambridge O Level Chemistry)

Use your copy of the Periodic Table to help you answer these questions.

Predict the formula of each of the following compounds.

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Deduce the formula of each of the following ions.

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Draw a diagram showing the arrangement of the valency electrons in one molecule of the covalent compound carbon dioxide.

Use o to represent an electron from a carbon atom.
Use x to represent an electron from an oxygen atom.

The law of constant composition states that all pure samples of a compound contain the same elements in the same proportion by weight.

A typical experiment to test this law is to prepare the same compound by different methods and then show that the samples have the same composition.

Methods of making copper(II) oxide include:

• heating copper carbonate,
• heating copper hydroxide,
• heating copper nitrate,
• heating copper foil in air.

Complete the following equations.

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Copper oxide can be reduced to copper by heating in hydrogen.

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The table below shows the results obtained by reducing the copper(II) oxide produced by different methods to copper.

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Iron pyrite, FeS₂, is known as Fool’s Gold because it is a shiny yellow solid which is similar in appearance to gold. Iron pyrite is an ionic compound. Gold is a metallic element.

Iron pyrite, FeS₂, contains positive and negative ions. The positive ion is Fe²⁺.
Deduce the formula of the negative ion.

A student is provided with a sample of iron pyrite and a sample of gold.

Suggest how the student could distinguish between the two substances.

Sulfur dioxide is produced on a large scale by heating iron pyrite strongly in air. The iron pyrite reacts with oxygen in the air producing iron(III) oxide, Fe₂O₃, and sulfur dioxide.

Construct a chemical equation for the reaction between iron pyrite and oxygen.

Element X can undergo the following physical changes.

Element X is a Group III metal. It burns in air to form an oxide X₂O₃.

Write a symbol equation for this reaction.

Aqueous silver nitrate is added to aqueous magnesium chloride.
A white precipitate forms.

Write an ionic equation for this reaction. Include state symbols.

Complete the chemical equation for the reaction of chlorine with phosphorus.

......Cl₂ + 2P → ......PCl₅

Aqueous potassium iodide reacts with aqueous copper(II) sulfate to produce iodine.

Balance the chemical equation for this reaction.

KI + CuSO₄ → CuI + I₂ + K₂SO₄

This question is about elements X, Y and Z.

An atom of element X is represented as

What is the electronic structure of the ion X^2–?

This question is about elements X, Y and Z.

Identify the atom against which the relative masses of all other atoms are compared.

Magnesium oxide, MgO, is formed when magnesium burns in oxygen.

Write the chemical equation for the reaction that occurs when magnesium burns in oxygen.

The structures of five organic compounds, A, B, C, D and E, are shown.

Answer the questions that follow.

Each letter may be used once, more than once or not at all.

Give the letter of the compound that has the empirical formula CH_2.

An endothermic reaction occurs when calcium nitrate is heated.

Balance the equation for this reaction.

.....Ca(NO₃)₂ → .....CaO + .....NO₂ + .....O₂

This question is about iron and iron compounds.

Iron reacts with chlorine to form iron(III) chloride, Fe₂Cl_6.

Balance the chemical equation for this reaction.

.....Fe + .....Cl₂ → Fe₂Cl₆

The structure of an ion is shown.

Deduce the molecular formula of this ion to show the number of iron, carbon and nitrogen atoms.

This question is about chlorine and compounds of chlorine.

Chlorine reacts with warm turpentine, C₁₀H₁₆.

Balance the chemical equation for this reaction.

C₁₀H₁₆ + 8Cl₂   → .....C + .....HCl

The equation shows the effect of heat on anhydrous zinc sulfate.  

 ZnSO₄  ZnO + SO₃

When 12.60 g of anhydrous zinc sulfate is heated, the mass of zinc oxide formed is 6.34 g.

Calculate the mass of zinc oxide formed when 63.0 g of anhydrous zinc sulfate is heated.

mass of zinc oxide = _______________

Complete the table to calculate the relative formula mass of anhydrous zinc sulfate, ZnSO₄. Use your Periodic Table to help you.

Quantities of chemicals, expressed in moles, can be used to find the formula of a compound, to establish an equation and to determine reacting masses.

A compound contains 72% magnesium and 28% nitrogen. What is its empirical formula?

A compound, Al₄C₃, contains only aluminium and carbon. 0.03 moles of this compound reacted with excess water to form 0.12 moles of Al(OH)₃ and 0.09 moles of CH₄.

Write a balanced equation for this reaction.

0.07 moles of silicon reacts with 25 g of bromine.

Si + 2Br₂ → SiBr₄

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A compound, X, contains 55.85% carbon, 6.97% hydrogen and 37.18% oxygen.

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Titanium is a transition element. It is isolated by the following reactions.

titanium ore → titanium(IV) oxide → titanium(IV) chloride → titanium

                              TiO₂                          TiCl₄                  Ti

Why is it usually necessary to include a number in the name of the compounds of transition elements?

Titanium(IV) chloride is made by heating the oxide with coke and chlorine.

TiO₂ + 2Cl₂ TiCl₄ + O₂

2C + O₂  2CO

Explain why the presence of coke ensures the maximum yield of the metal chloride.

Explain why the change, titanium(IV) chloride to titanium, is reduction.

The titanium ore contains 36.8% iron, 31.6% titanium and the remainder is oxygen.

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Across the world, food safety agencies are investigating the presence of minute traces of the toxic hydrocarbon, benzene, in soft drinks. It is formed by the reduction of sodium benzoate by vitamin C.

Sodium benzoate is a salt, it has the formula C₆H₅COONa. It can be made by the neutralisation of benzoic acid by sodium hydroxide.

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Benzene contains 92.3% of carbon and its relative molecular mass is 78.

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The structural formula of Vitamin C is drawn below.

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Fluorine forms both ionic and covalent compounds.

Magnesium reacts with fluorine to form the ionic compound magnesium fluoride.

The electronic structures of an atom of magnesium and an atom of fluorine are shown.

What is the formula of magnesium fluoride?

When copper is reacted with hot concentrated sulfuric acid, sulfur dioxide gas is formed.
Balance the chemical equation for this reaction.

Cu + .......H₂SO₄ → CuSO₄ + SO₂ + .......H₂O

When sulfuric acid reacts with ammonia the salt produced is ammonium sulfate.

Write the chemical equation for this reaction.   

Barium sulfate is an insoluble salt.
Barium sulfate can be made from aqueous ammonium sulfate using a precipitation reaction.

Write an ionic equation for this precipitation reaction. Include state symbols.

Oxygen is produced by the decomposition of hydrogen peroxide. Manganese(IV) oxide is the catalyst for this reaction.

Oxygen can also be produced by the decomposition of potassium chlorate(V), KClO₃.

The only products of this decomposition are potassium chloride and oxygen.

Write a chemical equation for this decomposition.

This question is about titanium.

Titanium is a transition metal that is extracted from titanium dioxide in a two-stage industrial process.

In the first stage, titanium dioxide reacts with carbon and chlorine to form titanium tetrachloride and carbon monoxide.

Write the balanced symbol equation for this reaction.

Explain how your chemical symbol equation in part (a) demonstrates the law of conservation of mass.

Identify two hazards associated with Stage 1.

Calculate, to three significant figures, the percentage by mass of chlorine in titanium tetrachloride. 

(A_r:  Ti = 48   Cl = 35.5)

Magnesium displaces copper from copper sulfate solution.

Write the balanced symbol equation for the reaction.

You should include state symbols. 

State two changes that would be observed during the displacement reaction.

Write the ionic equation for the displacement of copper from copper sulfate by magnesium. You should include state symbols.

Displacement reactions are examples of redox reactions.

Explain why the displacement reaction between magnesium and copper sulfate is both reduction and oxidation.

This question is about lead nitrate.

Lead nitrate is an ionic compound. 

It consists of Pb²⁺ and NO₃^- ions. 

What is the chemical formula of lead nitrate?

Calculate the percentage by mass of oxygen in lead nitrate, Pb(NO₃)₂.

A displacement reaction occurs between solutions of lead nitrate and potassium iodide, KI, to form solid lead(II) iodide and aqueous potassium nitrate.

Write the balanced symbol equation, including state symbols, for this reaction.

Table 1 summarises the solubility of a selection of ionic compounds in water.

Table 1

The displacement reaction between lead nitrate and potassium iodide forms a yellow precipitate.

Justify which chemical is responsible for the yellow colour.

The relative atomic mass, A_r, for each element is shown in the Periodic Table. 

Define the term relative atomic mass. 

The element gallium has a relative atomic mass of 69.735 and only contains two isotopes.

A sample of gallium contained the isotope ⁶⁹Ga, with a relative abundance of 63.25 %.

Calculate the mass number of the other isotope.
You must show all your working.

Define the term empirical formula.

An unknown compound contains carbon, hydrogen and oxygen.

It consists of 18% carbon,and 73 % oxygen.

Calculate the empirical formula of the unknown compound

Until recently, arsenic poisoning, either deliberate or accidental, has been a frequent cause of death. The symptoms of arsenic poisoning are identical with those of a common illness, cholera. A reliable test was needed to prove the presence of arsenic in a body.

In 1840, Marsh devised a reliable test for arsenic

Hydrogen is formed in this reaction. Any arsenic compound reacts with this hydrogen to form arsine which is arsenic hydride, AsH₃.

Another hydride of arsenic has the composition below.

arsenic 97.4% hydrogen 2.6%

In the 19th Century, a bright green pigment, copper(II) arsenate(V) was used to kill rats and insects. In damp conditions, micro-organisms can act on this compound to produce the very poisonous gas, arsine.