GCSE · Chemistry · AQA · Spec 8462
Conservation of mass and balanced equations
A chemical reaction can rearrange atoms into completely different substances — but it cannot make an atom disappear.
Chemistry · Reaction balancer
Balance atoms — not formulas
Adjust multipliers until each atom count matches on both sides. Never change a subscript to make it balance.
2H₂ + O₂ → 2H₂O: the multipliers change the number of whole formulas; the subscripts inside H₂, O₂ and H₂O stay fixed.
Balance without changing a formula
Problem
Balance H₂ + O₂ → H₂O.
Find relative formula mass
Problem
Calculate Mr for H₂O and CO₂ using Ar(H)=1, Ar(C)=12 and Ar(O)=16.
See conservation numerically
Problem
Use 2H₂ + O₂ → 2H₂O to show that the sum of Mr values in the amounts shown is the same on both sides. Use Ar(H)=1 and Ar(O)=16.
Percentage by mass
Problem
Calculate the percentage by mass of oxygen in MgO using Ar(Mg)=24 and Ar(O)=16.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- No atoms are lost or made during a chemical reaction, so total mass is conserved.
- A balanced symbol equation has the same number of each type of atom on both sides.
- A coefficient multiplies the whole formula; a subscript belongs inside the formula. Change coefficients to balance — never subscripts.
- Mr = the sum of the Ar values of all atoms shown in the formula.
- For the amounts shown in a balanced equation, total relative mass of reactants = total relative mass of products.
- Percentage by mass = (Ar × number of atoms of that element ÷ Mr) × 100.
The big picture
Mass is conserved in a chemical reaction because atoms are rearranged, not created or destroyed. Balanced symbol equations therefore contain the same number of each type of atom on both sides. Balance equations by changing coefficients in front of formulae, never the subscripts inside them. Relative formula mass, Mr, is found by adding the Ar values for all atoms shown in a formula. For the amounts shown by a balanced equation, the total relative masses on the reactant and product sides are equal. Percentage by mass is (Ar × number of that atom ÷ Mr) × 100.
Key points
Worked example
Problem
A compound has formula CO₂. Using Ar(C)=12 and Ar(O)=16, calculate its Mr and the percentage by mass that is oxygen.
⚠ Watch out
Changing a subscript to balance an equation. That changes the substance's formula. Keep every formula fixed and change only the coefficients in front.
★ Exam tip
For AQA Chemistry, keep balancing at this level to atom counting and coefficients. Do not bring in moles, limiting reactants, half equations or ionic equations on this page.
Memory hook
Atoms are the inventory: you may rearrange the stock, but the count of each element must balance before and after.
Check yourself
Using Ar(H)=1 and Ar(O)=16, calculate Mr of H₂O and the percentage by mass of oxygen in H₂O.
Flashcards
(14)What does conservation of mass mean in a chemical reaction?
What must be true of each element in a balanced symbol equation?
What does a coefficient in front of a chemical formula do?
What does a subscript inside a chemical formula tell you?
When balancing an equation, should you change coefficients or subscripts?
Balance: H₂ + O₂ → H₂O
Why is 2H₂ + O₂ → 2H₂O balanced?
How is relative formula mass, Mᵣ, calculated?
Using Aᵣ(H) = 1 and Aᵣ(O) = 16, what is Mᵣ of H₂O?
Do relative atomic mass Aᵣ and relative formula mass Mᵣ have units?
In a balanced equation, how should the total relative formula masses of the shown reactant amounts compare with the products?
For 2H₂ + O₂ → 2H₂O, using Aᵣ(H)=1 and Aᵣ(O)=16, what relative masses are shown on each side?
What is the formula for percentage by mass of an element in a compound?
Using Aᵣ(H)=1 and Aᵣ(O)=16, what percentage by mass of H₂O is oxygen?
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