Key Terms
- Chemical formula: Type and number of atoms per unit
- Coefficient: Large number in front of formula (balancing)
- State symbol: (s), (l), (g), (aq)
- Conservation of mass: Total mass reactants = total mass products
Diatomic Elements
- H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂
- Mnemonic: Have No Fear Of Ice Cold Beer
Common Ions
- NH₄⁺ (ammonium), OH⁻ (hydroxide)
- NO₃⁻ (nitrate), CO₃²⁻ (carbonate)
- SO₄²⁻ (sulfate), PO₄³⁻ (phosphate)
Must-Know Facts
- Never change subscripts — only coefficients
- Gas escape → apparent mass decrease
- Gas absorbed → apparent mass increase
- Swap-and-drop for ionic formulae
- Brackets if >1 polyatomic ion needed
Key Equations
- Conservation of mass: total mass reactants = total mass products
- Mr(compound) = sum of all Ar values in the formula
- Relative formula mass has no units
Common Mistakes
- Changing subscripts to balance equations: Only change the coefficients (large numbers in front) — changing subscripts changes the substance entirely
- Forgetting mass appears to decrease when gas escapes: The total mass is conserved — gas has mass too, it just escapes to the surroundings
- Wrong ionic formulae without brackets: When more than one polyatomic ion is needed (e.g. Ca(OH)₂), brackets are essential around the ion
- Not including state symbols: (s), (l), (g), (aq) are required for full marks in many equation questions
This topic summary covers Knowledge Organiser: Formulae, Equations and Conservation of Mass within Formulae, Equations and Conservation of Mass for GCSE Chemistry. Revise Formulae, Equations and Conservation of Mass in Quantitative Chemistry for GCSE Chemistry with 13 exam-style questions and 12 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 14 of 14 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Formulae, Equations and Conservation of Mass
Which of the following is the correct formula for oxygen gas?
Explain why mass is conserved in a chemical reaction.
Quick recall flashcards
State the law of conservation of mass.
In a chemical reaction, the total mass of the products equals the total mass of the reactants.
No atoms are created or destroyed — they are only rearranged.
Name the seven diatomic elements.
H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂
Mnemonic: Have No Fear Of Ice Cold Beer