The Nature of Substances & Chemical Reactions (WJEC GCSE Science (Double Award): Chemistry): Exam Questions

A sample of an oxide of nitrogen contains 1.4 g of nitrogen and 4.0 g of oxygen.

Find the simplest formula of this oxide. You must show your working.

A_r(N) = 14 A_r(O) = 16

Simplest formula .......................................................................

Rock salt is a mixture of salt and sand. Crystals of pure salt can be obtained from rock salt.

A–E are the steps in the method used but they are in the wrong order.

Put the steps in the correct order. The first step is already included.

C → ...... → ...... → ...... → ......

A student was asked to investigate the dyes present in an orange sweet.

The student carried out the following method. There are two errors in the method.

• Draw a line using a ruler and pen on chromatography paper.

• Place a sample of the orange colour on the line.

• Stand the chromatography paper in a beaker and add enough water to just cover the sample.

• Leave the paper to stand until the water rises to the top of the paper.

(i) State the two errors in the method.

1.

2.

[2]

(ii) Another student used a correct method and obtained the chromatogram below.

Use the formula to calculate the R_f value for pigment 2.

$R_f=\frac{\text{distance travelled by the pigment}}{\text{distance travelled by the solvent front}}$

R_f = .......................................................

[2]

Lithium reacts with oxygen. The word equation for this reaction is as follows.

lithium + oxygen → lithium oxide

(i) Name the metal in this equation.

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

[1]

(ii) Name the compound in this equation.

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

[1]

(iii) Name a reactant in this equation.

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

[1]

(iv) Lithium oxide contains the ions Li⁺ and O^2-.

Underline the formula of lithium oxide.

LiO₂ 2LiO Li₂O Li²O

[1]

Compounds are formed when atoms of different elements combine.

Draw one line from each compound to its formula.

Magnesium oxide contains the ions Mg²⁺ and O^2-.

Give the formula of magnesium oxide.

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

Calcium sulfate has the formula CaSO₄.

Calculate the relative formula mass (M_r) of calcium sulfate.

A_r(Ca) = 40 A_r(S) = 32 A_r(O) = 16

M_r = .........................................................

Aluminium chloride, AlCl₃, is formed by the reaction of aluminium with chlorine.

Write a number in the box to balance the equation for this reaction.

2Al + [ ] Cl₂ → 2AlCl₃

The diagram shows the separation of a mixture of liquids J and K.

• The temperature on the thermometer stays at 56 °C even though the mixture is still being heated

• Drops of liquid J fall steadily into the flask

Use the information given above and in the diagram.

State the conclusions that you can draw about the boiling points of liquids J and K.

K is a compound with the formula C₄H₈O. The relative formula mass (M_r) of compound K is 72.

Calculate the percentage by mass of carbon in compound K.

A_r(C) = 12

Percentage = ....................................................................... %

The diagrams show three methods, A, B and C, used to separate mixtures.

Give the letter, A, B or C, of the method you would use to:

(i) remove sand from sea-water, .....................................................

[1]

(ii) show that sea-water contains salt. .....................................................

[1]

Paper chromatography was used to show the pigments present in different hair dyes.

(i) Give the two dyes that are mixed to make brown hair dye.

.......................................................... and ..........................................................

[1]

(ii) Green hair dye is made by mixing together blue and yellow dyes.

Draw on the diagram the result that you would expect to see for green hair dye.

[1]

(iii) The Rf value of a substance can be used to identify that substance.

The Rf value of the pigment in red hair dye is 0.4.

Use the equation below to calculate the distance this red hair dye would have moved on the diagram.

distance moved = Rf × distance moved by solvent

distance moved = ............................................. cm

[2]

Lithium reacts with oxygen to give lithium oxide.

Complete the equation for the reaction by:

• giving the formula for lithium oxide

• putting numbers in the boxes to balance the equation

[ ] Li + O₂ → [ ] .....................

Iron reacts with chlorine gas to form iron(III) chloride according to the equation shown.

2Fe + 3Cl₂ → 2FeCl₃

Calculate the mass of iron(III) chloride that could be produced using 22.4 g of iron.

A_r(Fe) = 56 A_r(Cl) = 35.5

Mass = ....................................................................... g

White phosphorus has the formula P₄. It reacts with chlorine to form phosphorus trichloride, PCl₃.

Put a number in each box to balance the equation for this reaction.

P₄ + [ ] Cl₂ → [ ] PCl₃

The chemical formula of potassium carbonate is K₂CO₃.

(i) State how many carbon atoms are present in the formula, K₂CO₃. ..........................

[1]

(ii) Give the total number of atoms shown in the formula, K₂CO₃. ..........................

[1]

Calculate the percentage by mass of carbon in potassium carbonate, K₂CO₃.

percentage by mass = ....................................... %

Sodium hydrogencarbonate contains the HCO₃^- ion.

Sodium hydrogencarbonate is the only product made when captured carbon dioxide reacts with sodium hydroxide.

Write a balanced symbol equation for this reaction.

Low-grade copper ores commonly contain the mineral cuprite. Cuprous oxide is an old name for the oxide of copper found in cuprite.

A sample of cuprous oxide was found to contain 2.54 g of copper and 0.32 g of oxygen.

Use this information to find the simplest formula of cuprous oxide.

You must show your working.

A_r(Cu) = 63.5 A_r(O) = 16

Simplest formula .....................................................

Potassium nitrate decomposes on heating to produce potassium nitrite, KNO₂, and oxygen.

(i) Balance the equation for the reaction.

2KNO₃ → [ ]KNO₂ + O₂

[1]

(ii) On heating potassium nitrate, a student expected to make 4.35 g of potassium nitrite. However, in an experiment she only made 3.13 g.

Calculate the percentage yield of potassium nitrite in her experiment. Give your answer to the nearest whole number.

Percentage yield = ....................................................... %

[2]

The equation for the reaction between calcium carbonate and dilute hydrochloric acid is as follows.

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Calculate the mass of calcium chloride produced when 7.8 g of calcium carbonate is reacted with excess acid.

M_r(CaCO₃) = 100 M_r(CaCl₂) = 111

Mass = ........................................................ g

When the reaction was repeated with a different mass of calcium carbonate, it was found that 14.3 g of calcium chloride formed. This represents a yield of 53.7%.

Calculate the maximum mass of calcium chloride that could have been formed.

Maximum mass = ........................................................ g

Chlorine can form a number of compounds by reacting with oxygen gas. One of these is used in water treatment.

A sample of this compound contains 0.71 g of chlorine and 0.64 g of oxygen. Calculate its simplest formula.

Simplest formula ..........................................................

One of the diagrams represents the element oxygen. Oxygen reacts with potassium to form potassium oxide.

Give the formula for potassium oxide and balance the equation for this reaction.

[ ]K + O₂ → 2 ......................................

Fluorine reacts with titanium at very high temperatures, forming titanium fluoride.

In an experiment 3.8 g of fluorine reacted with 2.4 g of titanium.

Calculate the simplest formula for titanium fluoride. You must show your working.

A_r(Ti) = 48 A_r(F) = 19

Simplest formula ...................................................................................

A litre bottle of mineral water contains 184 mg of dissolved calcium sulfate, CaSO₄.

Calculate the number of moles of calcium sulfate present.

Give your answer to three significant figures.

1 mg = 0.001 g

A_r(Ca) = 40 A_r(S) = 32 A_r(O) = 16

Moles of calcium sulfate = ........................................... mol

(i) Lead can form a number of oxides with different formulae. Red lead, Pb₃O₄, is used to make batteries. It is manufactured by reacting lead carbonate with oxygen. Carbon dioxide is also produced.

Balance the equation for this reaction.

[ ]PbCO₃ + O₂ → [ ]Pb₃O₄ + [ ]CO₂

[1]

(ii) 11.36 g of a lead oxide contains 10.18 g of lead. Calculate the empirical formula of this oxide.

A_r(Pb) = 207 A_r(O) = 16

Empirical formula ...............................................

[3]

A teacher showed her class how copper can be extracted from copper(II) oxide using charcoal.

2CuO(s) + C(s) → CO₂(g) + 2Cu(s)

In her experiment, the teacher heated 1.59 g of copper(II) oxide and 0.12 g of charcoal in a boiling tube. She continued to heat the mixture strongly until it had been glowing for 5 minutes.

She recorded the mass of the boiling tube and its contents before heating and then again after heating, once the carbon dioxide produced had been released from the tube.

Mass of boiling tube = 37.43 g

Mass of boiling tube and contents before heating = 39.14 g

Mass of boiling tube and contents after heating = 38.82 g

(i) The teacher had expected the reaction to produce 1.27 g of copper from the masses of copper(II) oxide and charcoal used. Use her results to show that the mass produced suggests a 109% yield.

[2]

(ii) Assuming the reactants had been heated at a high enough temperature for sufficient time, suggest one reason for the yield being larger than expected.

[1]

Write the symbol equation for the reaction that takes place.

.................. + ........................ → ........................ + .......................

What is the correct formula for chlorine gas? Tick ($✓$) the correct answer.

The balanced symbol equation for the reaction between sodium and oxygen is given below.

4Na + O₂ → 2Na₂O

Calculate the mass of sodium oxide that should be produced when 0.736 g of sodium is burned in air.

mass = ............................................ g

When the reaction was carried out using 0.736 g of sodium, it was found that the mass of sodium oxide formed was 0.338 g.

(i) Use your answer to part (a) to calculate the percentage yield of the reaction.

percentage yield = .............................................. %

[1]

(ii) If no product was lost, suggest one possible explanation for the percentage yield not being 100%.

[1]

A food colouring contains a mixture of yellow and green dyes.

The table shows the R_f values of these dyes using different solvents in separate chromatography experiments.

Why would yellow and green dyes be difficult to separate in a chromatography experiment using water as the solvent?

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

Describe what you would expect to see in the chromatogram obtained using alcohol as the solvent.

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

Complete the chromatogram to show the results you would expect using acid as the solvent.

A year 10 class investigated the reactions between some metals and hydrochloric acid. Their results are summarised in the table below.

(i) State which metal did not react with hydrochloric acid.

Give a reason for your choice.

Metal

Reason

[1]

(ii) Calculate the final temperature for the reaction between magnesium and hydrochloric acid.

Final temperature = ........................................................ °C

[1]

What name is given to a reaction which gives a rise in temperature?

Choose your answer from the box.

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

Beryllium is found in the compound beryllium fluoride, BeF₂.

Give the total number of atoms shown in the formula.

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

Nitrogen can be obtained by heating sodium azide, NaN₃. Sodium is also produced in the reaction.

Complete the balancing of the equation for this reaction.

2NaN₃ → 2Na + [ ]N₂

Ionic compounds dissolve in water to form positive and negative ions.

Which two compounds may have dissolved in the drinking water at location B?

Put a tick (✓) in the box next to the correct answer.

(i) Calculate the relative formula mass (M_r) of calcium sulfate, CaSO₄.

A_r(Ca) = 40 A_r(S) = 32 A_r(O) = 16

M_r = .......................................................

[1]

(ii) Calculate the percentage by mass of fluorine in calcium fluoride, CaF₂.

M_r(CaF₂) = 78 A_r(F) = 19

Percentage = ....................................................... %

[2]

The diagram shows the reaction of sodium with chlorine.

(i) State why it is necessary to carry out this reaction in a fume cupboard.

[1]

(ii) Complete and balance the equation for the reaction of sodium and chlorine.

[ ]Na + Cl₂ → ......................................

[2]

The stock solution of sodium thiosulfate used in both experiments was made by dissolving 1.0 g of the solid in 1 dm³ of water.

Calculate the number of moles of sodium thiosulfate (Na₂S₂O₃) in 1.0 g.

Give your answer to two significant figures.

A_r(Na) = 23 A_r(S) = 32 A_r(O) = 16

Number of moles = ........................................................ mol

The combustion of the fossil fuel propane (C₃H₈) produces carbon dioxide and water. Balance the equation for this reaction.

C₃H₈ + [ ] O₂ → [ ]CO₂ + [ ]H₂O

Sodium reacts with water to produce sodium hydroxide and hydrogen. The equation for this reaction is shown.

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Calculate the mass of sodium needed to produce 11.2 g of hydrogen gas.

A_r(Na) = 23 A_r(H) = 1

Mass of sodium = ........................................................ g

Group 2 metals react in a similar way with water as Group 1 metals.

The word equation for the reaction of calcium and water is shown.

calcium + water → calcium hydroxide + hydrogen

Write the balanced symbol equation for this reaction.

Diagrams A, B, C and D represent hydrogen (H₂), sulfur dioxide (SO₂), hypochlorous acid (HClO) and phosphine (PH₃) but not in that order.

(i) Give the letter of the diagram that represents an element. Give a reason for your answer.

Letter ................................................

Reason ............................................................................................................

[2]

(ii) Give the letter of the diagram that represents SO₂.

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

[1]

(iii) Use diagrams A, B, C and D above to work out which of the following diagrams represents water (H₂O). Circle the correct diagram.

[1]

(i) Calculate the relative molecular mass (M_r) of hypochlorous acid, HClO.

A_r(H) = 1 A_r(O) = 16 A_r(Cl) = 35.5

M_r = .......................................................

[1]

(ii) The relative molecular mass (M_r) of sulfur dioxide is 64.

Calculate the percentage by mass of sulfur in sulfur dioxide, SO₂.

A_r(S) = 32

Percentage = ....................................................... %

[2]

Phosphine (PH₃) is made when phosphorus and hydrogen react.

Put a tick ($✓$) in the box next to the correctly balanced symbol equation for this reaction.

A student investigates the pigments found in some fruit and vegetables.

She obtains some coloured extracts from carrots, tomatoes and beetroot.

She places a spot of each extract on chromatography paper, along with spots of the three pigments beta-carotene, chlorophyll and betalain.

The diagram shows the chromatogram at the end of the experiment.

Give the names of all the substances which are not mixtures. Give a reason for your choice.

Substances ...................................................................................................................................................................

Reason ...................................................................................................................................................................

Put a tick ($✓$) in the boxes next to the two conclusions that can be drawn from the chromatogram.

One of the pigments present in the carrot extract has travelled 4.4 cm above the start line.

The solvent front has travelled 8.0 cm. Calculate the $R_f$ value of the pigment using the following equation.

$R_f=\frac{\text{distance travelled by pigment}}{\text{distance travelled by solvent front}}$

$R_f$ = .......................................................

Give the reason why there is a spot remaining on the start line in the chromatogram for beetroot.

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

Sodium reacts explosively with sulfuric acid. Sodium sulfate and hydrogen are produced. Complete and balance the equation for this reaction.

[ ]Na + H₂SO₄ → .................... + H₂

Ammonia present in the Earth's early atmosphere reacted with oxygen to produce nitrogen and water vapour. Complete the balancing of the symbol equation for this reaction.

[ ] NH₃ + 3O₂ → [ ]N₂ + 6H₂O

Complete and balance the symbol equation for the reaction between magnesium chloride and silver nitrate.

MgCl₂ + [ ]AgNO₃ → .....................................

0.103 g of silver nitrate, AgNO₃, was used to make up a solution.

Calculate the number of moles of silver nitrate in this mass. Give your answer in standard form.

A_r(Ag) = 108 A_r(N) = 14 A_r(O) = 16

Number of moles = ............................................ mol

One molecule of liquid E contains two oxygen atoms. The percentage by mass of oxygen in liquid E is 43.2%.

Use the following equation to calculate the relative molecular mass ($M_r$) of liquid E.

$\frac{massofoxygen}{M_r}\times 100=43.2$

$A_r\left(O\right)=16$

$M_r$ = .......................................................

The symbol equation for the reaction between iron and chlorine is as follows.

2Fe + 3Cl₂ → 2FeCl₃

Calculate the mass of chlorine needed to react with 1.32 g of iron.

A_r(Fe) = 56 A_r(Cl) = 35.5

Mass of chlorine = ....................................................... g

(i) Under certain conditions, Group 7 elements will react with each other to produce new compounds.

When chlorine is reacted with bromine, chlorine tribromide is made.

Balance the symbol equation for this reaction.

Cl₂ + [ ]Br₂ → ....[ ]ClBr₃

[1]

(ii) A chemist calculated that if she reacted 7.00 g of chlorine with an excess of bromine, the theoretical mass of chlorine tribromide produced is 27.55 g.

However, when she carried out the experiment using 7.00 g of chlorine the mass of chlorine tribromide obtained was 21.34 g.

Calculate the percentage yield of chlorine tribromide.

Percentage yield = ....................................................... %

[1]

Hydrogen peroxide solution decomposes to give water and oxygen gas. Many metal oxides act as catalysts for the reaction.

Use the symbol equation for the reaction to find the formula of hydrogen peroxide.

2 .............. → 2H₂O + O₂

Some of the impurities are removed from the furnace as slag. Slag contains calcium silicate which has the chemical formula CaSiO₃.

Tick (✓) the box next to the calculation used to find the correct relative formula mass of calcium silicate, CaSiO₃.

A_r(Ca) = 40 A_r(Si) = 28 A_r(O) = 16

820 tonnes of iron was extracted from 1 750 tonnes of iron ore. Calculate the percentage of iron in the ore. Give your answer to one decimal place.

Percentage = ....................................................... %