How do we write balanced equations and use the mole to work out masses?
Writing chemical formulae, constructing word and balanced symbol equations with state symbols, relative formula mass, the mole, the relationship between moles, mass and relative formula mass, and using balanced equations to calculate reacting masses.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on chemical equations and the mole, covering writing formulae and balanced symbol equations with state symbols, relative formula mass, the mole, and using equations to calculate reacting masses.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
CCEA Double Award wants you to write formulae and balanced symbol equations with state symbols, to work out relative formula mass, to understand the mole, and to use the mole and a balanced equation to calculate reacting masses. The balancing and the mole calculations are the high-value skills.
Formulae and balanced equations
A chemical formula shows the type and number of atoms in a substance, such as H2O (two hydrogen, one oxygen). A balanced symbol equation must have the same number of each type of atom on both sides, because atoms are not created or destroyed.
You balance by putting big numbers in front of formulae (never change the small subscript numbers). State symbols show the physical states: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Relative formula mass and the mole
The key relationship is:
For example, the Mr of water (H2O) is (2 times 1) plus 16, which is 18, so 18 g of water is 1 mole.
Calculating reacting masses
To find how much of one substance reacts or is made, use the moles and the balanced equation:
- Work out the moles of the substance you know (mass divided by Mr).
- Use the ratio in the balanced equation to find the moles of the substance you want.
- Convert those moles back to mass (moles times Mr).
Examples in context
- Example 1. Why mass is conserved
- In any balanced equation the atoms are just rearranged, so the total mass of reactants equals the total mass of products. This is why a balanced equation must have equal atoms on both sides.
- Example 2. Scaling up a reaction
- A chemist who knows the moles of one reactant can use the equation to predict exactly how much product will form, which is essential in industry to avoid waste. The mole is the bridge between the equation and real masses.
- Example 3. Why the apparatus mass seems to change
- When a metal burns in air it gains mass, because oxygen atoms from the air join it. When a carbonate is heated it loses mass, because carbon dioxide gas escapes. In both cases mass is conserved overall once you count the gas - the balanced equation shows the atoms are only rearranged, not created or destroyed.
Try this
Q1. What is the relative formula mass of carbon dioxide, CO2? (C = 12, O = 16) [1 mark]
- Cue. 12 + (2 x 16) = 44.
Q2. How many moles are in 36 g of water (Mr = 18)? [1 mark]
- Cue. 36 divided by 18 = 2 moles.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style3 marksBalance the equation: Mg + O2 gives MgO. Show the balanced equation.Show worked answer →
Balance the atoms for three marks.
There are 2 oxygen atoms on the left in O2 but only 1 in MgO, so put a 2 in front of MgO.
That gives 2 magnesium on the right, so put a 2 in front of Mg on the left.
The balanced equation is 2Mg + O2 gives 2MgO. Markers reward the correct big numbers and equal atoms on both sides.
CCEA-style4 marksCalculate the mass of magnesium oxide formed when 48 g of magnesium burns completely. (Relative atomic masses: Mg = 24, O = 16.)Show worked answer →
Use moles and the equation 2Mg + O2 gives 2MgO for four marks.
Moles of Mg equal mass divided by relative atomic mass: 48 divided by 24 equals 2 mol.
From the equation, 2 mol Mg gives 2 mol MgO, so 2 mol of MgO is made.
Relative formula mass of MgO is 24 plus 16, which is 40.
Mass of MgO equals moles times relative formula mass: 2 times 40 equals 80 g. Markers reward the moles of Mg, the 1 to 1 ratio, and the final mass of 80 g.
Related dot points
- The structure of the atom in terms of protons, neutrons and electrons, their relative charges and masses, atomic number and mass number, isotopes, and calculating relative atomic mass from isotopic abundances.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on atomic structure, covering protons, neutrons and electrons with their charges and masses, atomic number and mass number, isotopes, and calculating relative atomic mass from isotopic abundances.
- Electron arrangement in shells for the first 20 elements, the link between outer electrons, group number and reactivity, and the modern organisation of the Periodic Table by atomic number into periods and groups with metals and non-metals.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on the Periodic Table and electron arrangement, covering electron shells for the first 20 elements, the link between outer electrons, group number and reactivity, and the organisation of the table into periods and groups.
- Acids, bases and alkalis in terms of hydrogen and hydroxide ions, the pH scale and indicators, neutralisation, the reactions of acids with metals, oxides, hydroxides and carbonates, and preparing soluble salts.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on acids, bases and salts, covering the pH scale and indicators, neutralisation, the reactions of acids with metals, oxides, hydroxides and carbonates, and preparing soluble salts.
- Ionic bonding as the transfer of electrons, covalent bonding as the sharing of electrons, metallic bonding as ions in a sea of delocalised electrons, drawing dot-and-cross diagrams, and how each type of structure explains properties.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on chemical bonding, covering ionic bonding by electron transfer, covalent bonding by sharing, metallic bonding, dot-and-cross diagrams, and how each structure explains the properties of substances.
- Elements, compounds and mixtures, the difference between physical and chemical change, and the separation techniques of filtration, crystallisation, simple and fractional distillation, and chromatography.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on elements, compounds and mixtures, covering the difference between physical and chemical change, and the separation techniques of filtration, crystallisation, distillation and chromatography.
Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)