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GCSE Chemistry · GCSEChapter 7 · Relative formula mass and conservation of mass
GCSE · Chapter 7

Relative formula mass and conservation of mass

By the end of this chapter you will be able to

  • Use the periodic table to find Ar, and add the Ar values to find Mr
  • Work out the percentage by mass of an element in a compound
  • Balance a symbol equation by counting atoms on each side
  • Find an empirical formula from the masses of the elements in a compound
  • Use the masses in a balanced equation to predict the mass of a product or gas

You should already know

  • Relative atomic mass from the periodic table (Chapter 1)
  • Formulae and atoms in formulae (Chapter 1)

Where this leads

  • The mole, reacting masses and limiting reactants (Chapter 8)
  • Concentration of solutions (Chapter 9)
  • Electrolysis and Faraday masses (Chapter 11)

GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Calculate relative formula mass (Mr) from relative atomic masses, Calculate the percentage by mass of an element in a compound, Explain conservation of mass, balance symbol equations and use state symbols, Calculate the empirical formula from masses or percentages, Use a balanced equation to find the mass of a product or a gas, and mass ratios.

Exercise P  Do you remember?

1
FWhat is 2 x 16? Write the number.
2
FWhat is 12 + 2 x 16? Write the number.
3
FWhat is 40 / 100 x 100? Write the number.
4
FHow many atoms of oxygen are in one molecule of CO2? Write the number.
5
FWhat is 23 + 35.5? Write the number.
7.1

Relative formula mass (Mr)

GCSE
Calculate relative formula mass (Mr) from relative atomic masses
Video lesson: Relative formula mass (Mr)Coming soon
Context. A chemist needs to know how much of each chemical to weigh out. “How heavy is one molecule of water compared with one atom of carbon?” she asks. The answer uses relative atomic masses from the periodic table, added together to give the relative formula mass.

The relative atomic mass (Ar) of an element is on the periodic table. The relative formula mass (Mr) of a compound is the sum of the Ar values of all the atoms in the formula. Relative masses have no units. Multiply each Ar by the number of atoms of that element, then add the results.

ElementRelative atomic mass (Ar)
H1
C12
N14
O16
Na23
Mg24
S32
Cl35.5
Ca40
Fe56
Cu64
Relative atomic masses used in this chapter (rounded)
CompoundWorkingMr
H2O2 x 1 + 1618
CO212 + 2 x 1644
CaCO340 + 12 + 3 x 16100
H2SO42 x 1 + 32 + 4 x 1698
Na2CO32 x 23 + 12 + 3 x 16106
Relative formula masses worked out from the Ar values

Key ideas

  • Mr = the sum of all the relative atomic masses in the formula.
  • Multiply each Ar by the number of atoms first, then add.
  • Relative masses have no units.
  • Brackets are not used here; write each formula out in full.
Worked example 7.1.1
Calculate the relative formula mass of H2O. Write the number.

Method. Multiply, then add.

  1. Hydrogen. There are 2 H atoms, so 2 x 1 = 2.
    2 x 1 = 2
  2. Oxygen. There is 1 O atom, so 1 x 16 = 16.
    16
  3. Add. Mr = 2 + 16.
    2 + 16 = 18
  4. Write the answer.
    18

Check. Water has Mr 18. ✓

Worked example 7.1.2
Calculate the relative formula mass of CaCO3. Write the number.

Method. Add the three elements, with the correct numbers of atoms.

  1. Calcium. 1 Ca atom: 1 x 40 = 40.
    40
  2. Carbon. 1 C atom: 1 x 12 = 12.
    12
  3. Oxygen. 3 O atoms: 3 x 16 = 48.
    48
  4. Add. 40 + 12 + 48.
    100
  5. Write the answer.
    100

Check. Calcium carbonate has Mr 100. ✓

Worked example 7.1.3
Calculate the relative formula mass of Na2CO3. Write the number.

Method. Use the number of atoms of each element.

  1. Sodium. 2 Na atoms: 2 x 23 = 46.
    46
  2. Carbon. 1 C atom: 12.
    12
  3. Oxygen. 3 O atoms: 3 x 16 = 48.
    48
  4. Add. 46 + 12 + 48 = 106.
    106
  5. Write the answer.
    106

Check. Sodium carbonate has Mr 106. ✓

Watch out

Watch out: multiply each Ar by the number of atoms before you add. A common mistake is to add 16 once for each O, when there are three O atoms in CaCO3.

Exam tip

Exam tip. Write the working as a list of elements and totals before you add. It makes it easy to check.

Exercise 7A  Relative formula mass

1
FWhat is the Ar of oxygen? Write the number.
2
FWhat is the Ar of sodium? Write the number.
3
FCalculate the Mr of CO2. Write the number.
4
FCalculate the Mr of MgO. Write the number.
5
FCalculate the Mr of NH3. Write the number.
6
FCalculate the Mr of CH4. Write the number.
7
FCalculate the Mr of Al2O3, with Ar of aluminium 27 and oxygen 16. Write the number.
8
FCalculate the Mr of MgCl2, with Ar of chlorine 35.5. Write the number.
9
FCalculate the Mr of H2SO4. Write the number.
10
FCalculate the Mr of CuSO4. Write the number.
11
PSCalculate the Mr of Fe2O3. Write the number.
12
PSA compound has Mr 98 and contains only H, S and O, with 4 O atoms. What is the Mr contributed by the 4 O atoms? Write the number.
13
RExplain why the Mr of a compound is found by adding the Ar values, in one sentence. Accept any sentence that says the Mr is the total mass of all the atoms in the formula.
14
RTwo pupils calculate the Mr of CO2. Pupil A writes 12 + 16 = 28. Pupil B writes 12 + 2 x 16 = 44. Which is correct? Write A or B.
15
RExplain why the Ar values can be decimals, such as Cl = 35.5, in one sentence. Accept any sentence that says the Ar is an average of the isotopes.
7.2

Percentage by mass

GCSE
Calculate the percentage by mass of an element in a compound
Video lesson: Percentage by massComing soon
Context. A fertiliser bag says it contains a certain percentage of nitrogen. “How is that figure worked out?” asks a farmer. The answer uses the Ar of nitrogen, multiplied by the number of atoms, and divided by the Mr of the compound.

The percentage by mass of an element in a compound is:

percentage = (Ar x number of atoms of the element) / Mr x 100

Use the Mr from section 7.1, and the number of atoms of the element in one formula.

CompoundElementWorkingPercentage by mass
MgOMg24 / 40 x 10060%
MgOO16 / 40 x 10040%
CaCO3O48 / 100 x 10048%
Fe2O3Fe112 / 160 x 10070%
CuOCu64 / 80 x 10080%
Percentage by mass of an element in some compounds

Key ideas

  • Percentage by mass = (Ar x number of atoms) / Mr x 100.
  • The Mr is the total mass of the compound; the Ar x number is the mass of the element in it.
  • Give the answer to an appropriate number of significant figures, usually one or two decimal places.
  • Check that the element percentages in a compound add up to 100 if you have them all.
Worked example 7.2.1
Calculate the percentage by mass of magnesium in MgO. Write the number.

Method. Use the formula.

  1. Mass of Mg. One Mg atom has mass 24.
    24
  2. Mr. The Mr of MgO is 24 + 16 = 40.
    40
  3. Percentage. 24 / 40 x 100 = 60.
    60
  4. Write the answer.
    60

Check. The oxygen is the other 40%, and 60 + 40 = 100. ✓

Worked example 7.2.2
Calculate the percentage by mass of carbon in CaCO3. Write the number.

Method. Find the mass of carbon, and the Mr.

  1. Mass of C. One C atom has mass 12.
    12
  2. Mr. The Mr of CaCO3 is 100.
    100
  3. Percentage. 12 / 100 x 100 = 12.
    12
  4. Write the answer.
    12

Check. Carbon is 12% of calcium carbonate by mass. ✓

Worked example 7.2.3
Calculate the percentage by mass of iron in Fe2O3. Write the number.

Method. Two Fe atoms contribute the iron mass.

  1. Mass of Fe. Two Fe atoms: 2 x 56 = 112.
    112
  2. Mr. The Mr of Fe2O3 is 160.
    160
  3. Percentage. 112 / 160 x 100 = 70.
    70
  4. Write the answer.
    70

Check. The oxygen makes up the other 30%. ✓

Watch out

Watch out: use the number of atoms of the element (for example 2 Fe in Fe2O3), not 1. Forgetting the number of atoms is the most common error.

Exam tip

Exam tip. Write the Mr first, then the mass of the element, then divide and multiply by 100. Show each line.

Exercise 7B  Percentage by mass

1
FWhat is the Mr of CuO? Write the number.
2
FWhat is the percentage by mass of oxygen in MgO? Write the number.
3
FWhat is the percentage by mass of calcium in CaCO3? Write the number.
4
FWhat is the percentage by mass of oxygen in CaCO3? Write the number.
5
FWhat is the percentage by mass of copper in CuO? Write the number.
6
FWhat is the percentage by mass of iron in Fe2O3? Write the number.
7
PSWhat is the percentage by mass of carbon in CO2, to one decimal place? Write the number.
8
PSWhat is the percentage by mass of hydrogen in H2O, to one decimal place? Write the number.
9
PSA compound is 60% magnesium by mass. If the Mr is 40 and the compound is MgO, what is the mass of magnesium in one formula unit? Write the number.
10
PSCalculate the percentage by mass of nitrogen in NH3, to one decimal place. Write the number.
11
RExplain why a compound with more oxygen by mass does not always have a higher Mr, in one sentence. Accept any sentence that says the percentage depends on the other elements as well as oxygen.
12
RTwo compounds are compared: MgO (Mg 60%) and CaCO3 (Ca 40%). Which contains the larger percentage by mass of its metal? Write the formula.
7.3

Conservation of mass and balancing equations

GCSE
Explain conservation of mass, balance symbol equations and use state symbols
Video lesson: Conservation of mass and balancing equationsComing soon
Context. A pupil burns a piece of magnesium ribbon in a closed container and weighs the container before and after. “The mass looks the same,” she says. The mass has not disappeared: the atoms are only rearranged, and no atoms are lost or made.

In a chemical reaction, the total mass of the reactants equals the total mass of the products. This is the law of conservation of mass. Atoms are rearranged, not created or destroyed. A balanced symbol equation has the same number of each type of atom on both sides.

Use state symbols in brackets: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.

before: sealed flask12 g magnesium+ 8 g oxygentotal: 20 greactafter: same flask20 g magnesium oxidetotal: 20 g
Conservation of mass: a sealed flask before and after a reaction (not to scale)
EquationAtoms on each sideResult
2Mg + O2 -> 2MgO2 Mg, 2 Obalanced
CH4 + 2O2 -> CO2 + 2H2O1 C, 4 H, 4 Obalanced
N2 + 3H2 -> 2NH32 N, 6 Hbalanced
Balanced equations: count the atoms on each side

Key ideas

  • Mass is conserved in a reaction; the total mass is the same before and after.
  • Balance by changing the numbers in front of formulae (coefficients), never the small numbers in formulae.
  • Count each element on both sides before you finish.
  • State symbols: (s), (l), (g) and (aq).
Worked example 7.3.1
Balance the equation: H2 + O2 -> H2O. Write the balanced equation.

Method. Count the atoms, then add numbers in front.

  1. Count. Left: 2 H and 2 O. Right: 2 H and 1 O.
    2 O on left, 1 O on right
  2. Balance O. Put 2 in front of H2O: now 2 O and 4 H on the right.
    2H2O
  3. Balance H. Put 2 in front of H2: now 4 H and 2 O on each side.
    2H2 + O2 -> 2H2O
  4. Write the answer.
    2H2 + O2 -> 2H2O

Check. Four H and two O on each side. ✓

Worked example 7.3.2
Balance the equation: Mg + O2 -> MgO. Write the balanced equation.

Method. Balance the oxygen first, then the magnesium.

  1. Count. Left: 1 Mg and 2 O. Right: 1 Mg and 1 O.
    2 O on left
  2. Balance. Put 2 in front of MgO: 2 Mg and 2 O on the right.
    2MgO
  3. Check. Put 2 in front of Mg: 2 Mg and 2 O on each side.
    2Mg + O2 -> 2MgO
  4. Write the answer.
    2Mg + O2 -> 2MgO

Check. Two Mg and two O on each side. ✓

Worked example 7.3.3
Write the state symbol for the solid formed in a reaction of sodium chloride solution with silver nitrate solution. Write the symbol. (The solid is silver chloride.)

Method. Decide the state of the product.

  1. Look. The silver chloride formed is a solid that comes out of solution.
    solid
  2. Decide. The state symbol for a solid is (s).
    (s)
  3. Write the answer.
    (s)

Check. The other products are aqueous. ✓

Watch out

Watch out: never change the small subscript numbers in a formula to balance an equation. For example, H2O must stay H2O; only the number in front (2H2O) can change.

Exam tip

Exam tip. Balance the equation, then count each element on both sides once more before you write the answer.

Exercise 7C  Conservation of mass and balancing equations

1
FWhat is the law of conservation of mass? Write the words.
2
FBalance the equation: Mg + O2 -> MgO. Write the number in front of Mg. Write the number.
3
FBalance the equation: Na + Cl2 -> NaCl. Write the number in front of NaCl. Write the number.
4
PSBalance the equation: CH4 + O2 -> CO2 + H2O. Write the number in front of O2. Write the number.
5
PSBalance: N2 + H2 -> NH3. Write the two numbers in front of H2 and NH3. Write two numbers, H2 then NH3.
6
FBalance: CaCO3 -> CaO + CO2. Is it already balanced? Write yes or no.
7
FWrite the state symbol for a gas. Write the symbol.
8
FWrite the state symbol for the liquid water. Write the symbol.
9
FWrite the state symbol for sodium chloride dissolved in water. Write the symbol.
10
PSBalance: Fe + CuSO4 -> FeSO4 + Cu. Is it balanced? Write yes or no.
11
RExplain why the mass of a reaction in a closed flask stays the same, in one sentence. Accept any sentence that says no atoms are lost or gained, so the total mass is conserved.
12
RA pupil says the mass of a burning candle decreases, so mass is not conserved. Explain why this is not a valid conclusion, in one sentence. Accept any sentence that says the products escape as gases, so the mass of the products in the flask is not measured.
7.4

Empirical formula

GCSE
Calculate the empirical formula from masses or percentages
Video lesson: Empirical formulaComing soon
Context. A pupil heats 5.6 g of iron in air and finds that the rust has a mass of 8.0 g. “How do I work out the formula of the rust?” she asks. The empirical formula gives the simplest whole-number ratio of the atoms in a compound.

The empirical formula is the simplest whole-number ratio of the atoms in a compound. To find it: divide the mass of each element by its Ar to get the ratio of moles of atoms, then divide each number by the smallest, and multiply to get whole numbers if needed.

Key ideas

  • Empirical formula = simplest whole-number ratio of atoms.
  • Step 1: divide the mass of each element by its Ar.
  • Step 2: divide each result by the smallest result.
  • Step 3: write the numbers as subscripts, if they are whole numbers.
Worked example 7.4.1
A compound contains 2.4 g of magnesium and 1.6 g of oxygen. Find its empirical formula. Write the formula.

Method. Divide each mass by the Ar, then find the simplest ratio.

  1. Magnesium. 2.4 / 24 = 0.1.
    0.1
  2. Oxygen. 1.6 / 16 = 0.1.
    0.1
  3. Ratio. The ratio is 0.1 : 0.1, which is 1 : 1.
    1 : 1
  4. Write the answer.
    MgO

Check. Magnesium oxide is MgO. ✓

Worked example 7.4.2
A compound contains 5.6 g of iron and 2.4 g of oxygen. Find its empirical formula. Write the formula.

Method. Use the Ar of each element.

  1. Iron. 5.6 / 56 = 0.1.
    0.1
  2. Oxygen. 2.4 / 16 = 0.15.
    0.15
  3. Ratio. Divide both by 0.05: 0.1 / 0.05 = 2 and 0.15 / 0.05 = 3, so 2 : 3.
    2 : 3
  4. Write the answer.
    Fe2O3

Check. Iron(III) oxide is Fe2O3. ✓

Worked example 7.4.3
A compound is 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Find its empirical formula. Write the formula.

Method. Treat the percentages as masses in 100 g, then divide by the Ar.

  1. Carbon. 40 / 12 = 3.33.
    3.33
  2. Hydrogen. 6.7 / 1 = 6.7.
    6.7
  3. Oxygen. 53.3 / 16 = 3.33.
    3.33
  4. Ratio. Divide by 3.33: C = 1, H = 2, O = 1.
    C : H : O = 1 : 2 : 1
  5. Write the answer.
    CH2O

Check. The formula of a sugar-type compound (empirical). ✓

Watch out

Watch out: the empirical formula is the simplest ratio, not the real formula of a molecule. Glucose is C6H12O6, but its empirical formula is CH2O.

Exam tip

Exam tip. Set out a table: element, mass, Ar, divide, ratio. Keep at least two decimal places until the last step.

Exercise 7D  Empirical formula

1
FWhat is the empirical formula? Write the words.
2
FA compound has 1 Mg for every 1 O. Write its empirical formula. Write the formula.
3
FDivide 2.4 g of magnesium by its Ar of 24. Write the answer.
4
FDivide 1.6 g of oxygen by its Ar of 16. Write the answer.
5
FAfter dividing each mass by its Ar, a compound gives 0.1 for magnesium and 0.1 for oxygen. What is the simplest ratio of Mg to O? Write the ratio.
6
FDivide 5.6 g of iron by its Ar of 56. Write the answer.
7
PSDivide 2.4 g of oxygen by 16, then divide the answer by 0.05. Write the number.
8
PSAfter dividing each mass by its Ar, a compound gives 0.1 for iron and 0.15 for oxygen. Find the formula. Write the formula.
9
PSA compound is 70% iron and 30% oxygen by mass. Find the ratio of moles of Fe to O, to the nearest whole ratio. Write the formula.
10
PSA compound is 40% carbon, 6.7% hydrogen and 53.3% oxygen. Find the empirical formula. Accept CH2O.
11
RExplain why a compound with the empirical formula CH2O might have a molecular formula of C6H12O6, in one sentence. Accept any sentence that says the molecular formula is a multiple of the empirical formula.
7.5

Using balanced equations to find masses

GCSE
Use a balanced equation to find the mass of a product or a gas, and mass ratios
Video lesson: Using balanced equations to find massesComing soon
Context. A school experiment heats 25 g of calcium carbonate and the gas that escapes is collected. “How much gas will there be?” asks a pupil. Conservation of mass and the balanced equation give the answer: the masses of the products add up to the mass of the reactant.

A balanced equation gives a mass ratio. For example, in 2H2 + O2 -> 2H2O, the masses are 2 x 2 = 4 g of hydrogen, 32 g of oxygen, and 36 g of water. To find the mass of a product from a given mass of a reactant, take the mass ratio from the equation and scale it to the mass you have.

Key ideas

  • Mass ratios come from the balanced equation, using Mr values.
  • In a reaction, the total mass of products equals the total mass of reactants.
  • A gas that escapes from an open container has a mass that you must calculate, not measure.
  • Check your answer: products must add up to the reactant mass when all products are counted.
Worked example 7.5.1
In 2H2 + O2 -> 2H2O, what mass of water is made from 4 g of hydrogen? Write the number in g.

Method. Use the masses from the balanced equation.

  1. Ratio. The equation makes 2 x 18 = 36 g of water from 4 g of hydrogen, with 32 g of oxygen.
    36 g
  2. Scale. The hydrogen mass is 4 g, which is exactly the mass in the equation.
    no scaling needed
  3. Write the answer.
    36

Check. Oxygen 32 g + hydrogen 4 g = 36 g. ✓

Worked example 7.5.2
When 25 g of calcium carbonate is heated, what mass of carbon dioxide is made? (CaCO3 -> CaO + CO2.) Write the number in g.

Method. Use the mass of CO2 per 100 g of CaCO3.

  1. Equation. 100 g of CaCO3 gives 44 g of CO2.
    44 g per 100 g
  2. Scale. 25 g is a quarter of 100 g, so 44 / 4 = 11 g.
    11 g
  3. Write the answer.
    11

Check. The mass of CaO is 25 - 11 = 14 g, which is 56 / 4. ✓

Worked example 7.5.3
In 2Mg + O2 -> 2MgO, 4.8 g of magnesium reacts. What mass of magnesium oxide is made? Write the number in g.

Method. Use the mass ratio of Mg to MgO from the equation.

  1. Ratio. The equation has 48 g of Mg giving 80 g of MgO.
    48 : 80
  2. Scale. So 4.8 g gives 4.8 x 80 / 48 = 8.0 g of MgO.
    8.0 g
  3. Write the answer.
    8

Check. Check: 8.0 g MgO - 4.8 g Mg = 3.2 g of oxygen, which is 32 x 4.8 / 48 = 3.2. ✓

Watch out

Watch out: a mass ratio uses the masses from the balanced equation (2 x Mr for 2 molecules), not the Mr of one molecule alone.

Exam tip

Exam tip. First write the balanced equation, then the mass of each substance in the equation, then scale to the mass given.

Exercise 7E  Using balanced equations to find masses

1
FIn 2H2 + O2 -> 2H2O, what is the total mass of hydrogen and oxygen used? Write the number in g.
2
FIn CaCO3 -> CaO + CO2, what is the mass of CO2 made from 100 g of CaCO3? Write the number in g.
3
FIn CaCO3 -> CaO + CO2, what is the mass of CaO made from 100 g of CaCO3? Write the number in g.
4
PSIn a reaction, 10 g of reactants give 9.5 g of product in an open container. What mass of gas probably escaped? Write the number in g.
5
PSUsing 2Mg + O2 -> 2MgO, what mass of Mg is needed to make 20 g of MgO? Write the number in g.
6
PSIn 2Mg + O2 -> 2MgO, what mass of oxygen reacts with 12 g of magnesium? Write the number in g.
7
PSIn CH4 + 2O2 -> CO2 + 2H2O, what is the total mass of the products from 16 g of methane? Write the number in g.
8
RExplain why the mass of a gas from a reaction can be calculated but may be hard to measure, in one sentence. Accept any sentence that says the gas escapes, so it is found from the mass ratio or by subtraction.
9
RA pupil says “The mass of product is always greater than the mass of reactant.” Explain why this is wrong, in one sentence. Accept any sentence that says the total mass is conserved, so the products cannot be heavier than the reactants.

Chapter summary

SkillMethod
MrThe sum of the Ar values of all the atoms in the formula; no units.
Percentage by mass(Ar x number of atoms) / Mr x 100.
Conservation of massThe total mass of the products equals the total mass of the reactants.
Balancing equationsChange only the numbers in front; the same atoms on both sides.
State symbols(s), (l), (g), (aq).
Empirical formulaDivide mass by Ar, divide by the smallest, write the simplest whole-number ratio.
Mass ratiosUse the masses in the balanced equation, then scale to the mass given.

Can you…?

  • I can calculate Mr from Ar values
  • I can work out the percentage by mass of an element in a compound
  • I can explain conservation of mass and balance symbol equations
  • I can write state symbols correctly
  • I can find an empirical formula from masses or percentages
  • I can use a balanced equation to find the mass of a product or a gas

Key terms

relative atomic mass (Ar)
the average mass of an atom of an element, compared with carbon-12
relative formula mass (Mr)
the sum of the relative atomic masses of all the atoms in a formula
conservation of mass
the total mass of the products equals the total mass of the reactants
balanced symbol equation
an equation with the same number of each atom on both sides
state symbol
a symbol showing the physical state: (s), (l), (g) or (aq)
empirical formula
the simplest whole-number ratio of the atoms in a compound

Exercise 7Z  Check-up

1
Calculate the Mr of CaCl2, with Ar of Cl = 35.5. Write the number.
2
Calculate the percentage by mass of oxygen in MgO. Write the number.
3
Calculate the percentage by mass of copper in CuO. Write the number.
4
Balance: Mg + HCl -> MgCl2 + H2. Write the number in front of HCl. Write the number.
5
Write the state symbol for copper(II) sulfate dissolved in water. Write the symbol.
6
A compound contains 1.6 g of oxygen and 2.4 g of magnesium. Divide each mass by its Ar. Write the two numbers, oxygen then magnesium.
7
Using CaCO3 -> CaO + CO2, what mass of CO2 is made from 50 g of CaCO3? Write the number in g.
8
Explain the law of conservation of mass, in one sentence. Accept any sentence that says the total mass of the products equals the total mass of the reactants.
9
Calculate the Mr of Na2CO3. Write the number.

Total 25 marks. Allow about 25 minutes.

Exercise 7X  Exam-style questions

1
FCalculate the Mr of Al2O3, with Ar of Al = 27 and O = 16. Write the number.[2 marks]
2
PSCalculate the percentage by mass of aluminium in Al2O3, to one decimal place. Write the number.[3 marks]
3
PSBalance the equation: Fe + O2 -> Fe2O3. Write the number in front of Fe. Write the number.[2 marks]
4
FWrite the state symbol of water vapour produced in a reaction. Write the symbol.[1 mark]
5
PSA compound contains 5.6 g of iron and 2.4 g of oxygen. Find its empirical formula. Write the formula.[3 marks]
6
PSUsing 2Mg + O2 -> 2MgO, what mass of magnesium oxide is made from 6.0 g of magnesium? Write the number in g.[3 marks]
7
RExplain why a reaction in an open container seems to lose mass when a gas escapes, in two sentences. Accept any answer that says the gas escapes and is not weighed, though the total mass of all the products is conserved.[3 marks]
8
PSA compound is 60% magnesium and 40% oxygen by mass. Find its empirical formula. Write the formula.[3 marks]
9
PSUsing CaCO3 -> CaO + CO2, a pupil heats 10 g of CaCO3. What mass of CO2 is made? Write the number in g to one decimal place.[2 marks]
10
RExplain why the balanced equation for methane burning is CH4 + 2O2 -> CO2 + 2H2O, in one or two sentences. Accept any answer that counts 1 C, 4 H and 4 O on each side.[3 marks]
← Chapter 6: Metallic bonding, alloys and nanoparticlesChapter 8: The mole, reacting masses and limiting reactants →

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