Chemical calculations: mass & equations
Chemical calculations: mass & equations
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Chemical calculations: mass & equations
Open by framing the lesson around a big GCSE idea: chemists trust calculations because atoms are conserved, so balanced equations let us connect reacting masses.
Learning goal
I can use conservation of mass and balanced equations to calculate reacting masses in GCSE chemistry questions.
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Why mass is conserved
In a chemical reaction, atoms are rearranged, not created or destroyed
This means the total mass of reactants = total mass of products
The idea is called the conservation of mass
If the mass seems to change, something has usually entered or left the system
Stress that conservation of mass applies because the number of each type of atom stays the same before and after the reaction.
Open and closed systems
In a closed system, all substances stay in the container
Measured mass stays the same during the reaction
In an open system, a gas can escape or enter
This can make the measured mass appear to change
A common misconception is that mass is not conserved when magnesium burns or when gas bubbles out. Clarify that the measured mass changes only because the system is open.
Example: gas escaping
Calcium carbonate decomposes:
CaCO3 → CaO + CO2
If this happens in an open container, CO2 escapes. The solid left behind has a smaller mass, but the total mass of all products is still equal to the starting mass.
You can link this to practical work where students heat a carbonate and see the mass of the remaining solid decrease.
Exam-style check
A student mixes hydrochloric acid with marble chips in a flask on a balance. The reading falls during the reaction.
The balance reading falls because CO2 leaves the flask
The reaction still obeys conservation of mass
In a sealed flask, the total mass would stay constant
Model how to answer a 2–3 mark Edexcel explanation clearly: name the gas, state that atoms are conserved, and refer to the system being open or sealed.
Why does the balance reading fall in an open flask reaction?
Gas escapes
Mass is destroyed
Atoms vanish
Solid gets lighter
Checks the key idea from slides 2-5: measured mass can fall in an open system because a gas leaves, but total mass is still conserved.
Open and closed systems
In a closed system, no substances leave the container, so the measured mass stays the same. In an open system, a gas such as CO2 can escape and make the reading fall.
Reinforces the distinction between measured mass and total mass, using the carbonate example already taught.
What a symbol equation shows
A symbol equation uses chemical formulae to show:
the reactants and products
the correct formula of each substance
the ratio of particles reacting
Example: 2H2 + O2 → 2H2O
Make clear that the large numbers in front are coefficients, while the small numbers in formulae are part of the substance itself.
Balanced equations
A balanced equation has the same number of each type of atom on both sides.
Example:
2Mg + O2 → 2MgO
Left: 2 Mg atoms, 2 O atoms
Right: 2 Mg atoms, 2 O atoms
Link balancing directly to conservation of mass: if atom numbers match, mass is conserved.
How to balance equations
Do not change the small numbers in a formula
Change only the large numbers in front
Count atoms on each side
Adjust coefficients until each element matches
Example:
H2 + Cl2 → 2HCl
Students often try to turn HCl into H2Cl or similar. Emphasise that changing subscripts changes the substance.
Common balancing errors
Wrong: Mg + O2 → MgO2
This changes the product formula
Correct: 2Mg + O2 → 2MgO
Always keep the formulae correct first, then balance the equation.
This is a key exam-technique point. Edexcel questions often punish correct arithmetic built on an incorrect formula.
Exam-style check
Balance this equation:
__ Fe + __ O2 → __ Fe2O3
Balanced equation:
4Fe + 3O2 → 2Fe2O3
The coefficients give the ratio 4 : 3 : 2.
Talk through a reliable balancing method: balance Fe first, then O, then scale to whole numbers.
When balancing equations, what are you allowed to change?
Subscripts only
Formulae only
Coefficients only
Atom symbols only
Targets the common error on slides 8-9: students must not change the small numbers in formulae.
Balancing equations
A balanced equation has the same number of each type of atom on both sides. To balance an equation, change the coefficients in front, but never change the subscripts in a formula.
Echoes the exact balancing method and notation distinction from slides 6-10.
From equations to mass ratios
Balanced equations give mole ratios, and these lead to mass relationships.
Example:
2Mg + O2 → 2MgO
This means:
2 moles of Mg react with 1 mole of O2
2 moles of MgO are formed
For Combined Science, keep this intuitive: coefficients tell us how much of each substance is needed relative to the others.
Using relative formula mass
To compare masses, use Ar and Mr.
For MgO:
Ar of Mg = 24
Ar of O = 16
Mr of MgO = 24 + 16 = 40
So 24 g of magnesium reacts with 16 g of oxygen to make 40 g of magnesium oxide.
This is the key bridge between balancing and reacting mass calculations. Use rounded GCSE values unless a question gives different ones.
A reacting mass method
Write the balanced equation
Find the Mr or Ar values needed
Use the equation to get the known mass ratio
Scale the ratio up or down
Check the answer is sensible and in the correct units
Encourage the student to write the mass ratio step clearly. This improves method marks even if arithmetic goes wrong later.
Worked example
2Mg + O2 → 2MgO
What mass of magnesium oxide forms when 6 g of magnesium burns?
24 g Mg produces 40 g MgO
6 g is 1/4 of 24 g
So MgO formed = 1/4 × 40 = 10 g
Point out the scaling shortcut before introducing formal algebra. It is often the fastest approach in Foundation and many Higher questions.
Exam-style question
Iron reacts with sulfur:
Fe + S → FeS
Calculate the mass of iron sulfide made from 11.2 g of iron.
Ar: Fe = 56, S = 32
56 g Fe produces 88 g FeS
11.2 g is 1/5 of 56 g
Mass of FeS = 17.6 g
This is a good moment to insist on writing the ratio line explicitly: 56 g Fe → 88 g FeS.
Finding an unknown reactant mass
The same method works backwards.
Example:
2Mg + O2 → 2MgO
If 20 g of magnesium oxide forms:
40 g MgO comes from 24 g Mg
20 g is 1/2 of 40 g
Magnesium needed = 12 g
Students often think the method changes when the unknown is on the left-hand side. Show that the ratio idea stays exactly the same.
In 2Mg + O2 → 2MgO, 24 g of Mg makes how much MgO?
16 g
24 g
40 g
48 g
Uses the mass ratio established on slides 11-14.
Reacting mass method
Start with the balanced equation and work out the needed Ar/Mr values. Then write a clear mass ratio and scale it to match the mass in the question.
Anchors the exam technique sequence from slides 13-16.
Conservation of mass questions
Sometimes no equation is needed.
If 8.0 g of oxygen reacts completely with magnesium to make 12.0 g of magnesium oxide:
mass of magnesium = 12.0 g - 8.0 g = 4.0 g
Use:
mass of reactants = mass of products
Distinguish clearly between pure conservation-of-mass arithmetic and equation-based reacting mass questions.
When the numbers do not match
If a measured mass is lower or higher than expected, think about the experiment.
Was the container open?
Did a gas escape?
Was a gas from the air added, such as oxygen?
Was the reaction complete?
A classic example is magnesium gaining mass when burned because oxygen from the air combines with it.
Exam technique that earns marks
Start with the balanced equation
Show the Ar/Mr values used
Write a clear mass ratio
Show each calculation step
Include the final answer with units
Explain that many Edexcel questions award method marks, so a tidy structure matters as much as the final answer.
Checking if an answer is sensible
Product mass should fit the equation and the masses used
In a closed system, total mass should stay the same
If a reactant combines with oxygen, the product may have a greater mass than the starting substance
If a gas escapes from an open system, the measured mass may fall
Encourage quick estimation before accepting a calculated answer. This helps catch arithmetic slips.
A full GCSE-style example
Calcium reacts with oxygen:
2Ca + O2 → 2CaO
Calculate the mass of calcium oxide made from 8.0 g of calcium.
Ar: Ca = 40, O = 16
80 g Ca produces 112 g CaO
8.0 g is 1/10 of 80 g
Mass of CaO = 11.2 g
You can ask the student to identify each method step from the previous slide as they read the solution.
Key ideas to remember
Conservation of mass means total mass stays constant
A balanced equation shows equal numbers of each atom on both sides
Use the equation and Ar/Mr values to build a mass ratio
Then scale the ratio to the mass in the question
This slide resolves the lesson's central question: balanced equations make mass calculations possible because atoms, and therefore mass, are conserved.
Why might magnesium gain mass when it burns?
Iron enters
Water leaves
Oxygen adds
Mass appears
Checks whether the student can apply conservation of mass thinking to a less obvious case from slide 18.
Exam technique and sensible checks
To earn marks, begin with the balanced equation and show the Ar/Mr values used. In a closed system, total mass stays constant, so your final answer should look sensible.
Combines the exam-technique slides with the final checking ideas so the student rehearses both process and judgement.
What is one thing you will remember from this lesson?
Any reflective answer is valid. Encourage the student to name one method step, example or idea that now feels secure.
What helped you understand these calculations best?
Any reflective answer is valid. Prompt the student to be specific about whether balancing, mass ratios, conservation of mass or exam structure helped most.
Which part still feels hardest?
Any reflective answer is valid. Use responses to plan follow-up practice, especially on balancing equations, open vs closed systems, or scaling mass ratios.
Glossary
conservation of mass - the idea that mass is not created or destroyed in a chemical reaction
closed system - a system where no substances can enter or leave during the reaction
open system - a system where substances, especially gases, can enter or leave
symbol equation - an equation written using chemical formulae for reactants and products
balanced equation - an equation with equal numbers of each type of atom on both sides
coefficient - the large number placed in front of a formula in an equation
subscript - the small number in a chemical formula, such as the 2 in O2
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Glossary
Ar - relative atomic mass, the average mass of an atom compared with carbon-12
Mr - relative formula mass, found by adding the relative atomic masses in a formula
reactant - a substance that is used up in a chemical reaction
product - a substance formed in a chemical reaction
mass ratio - the relationship between the masses of substances that react or are formed
complete reaction - a reaction in which the limiting reactant has all been used up
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