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K324 / 2027
Chemical Calculations

Lesson 2 of 6

Balance atoms, then account for ions

A reaction changes arrangement, not the total number of atoms.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can interpret equations and states.Syllabus

    Syllabus K324, 4.1(d). Interpret equations and states

  • I can construct balanced equations.Syllabus

    Syllabus K324, 4.1(e). Construct balanced equations

Balance atoms, then account for ions

A reaction changes arrangement, not the total number of atoms.

Write correct formulae first, then change coefficients until each element has the same atom count on both sides. Never alter a subscript to balance an equation. State symbols distinguish solid (s), liquid (l), gas (g) and aqueous solution (aq). Aqueous means dissolved in water, not simply liquid.

Worked exampleBalance combustion

Write the complete combustion of propane.

Try it first. Then show the worked stepsHide the worked steps
  1. Start C3H8(g) + O2(g) -> CO2(g) + H2O(l).
  2. Balance carbon: 3CO2. Balance hydrogen: 4H2O.
  3. Products contain ten O atoms, requiring 5O2.
Answer

C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(l). Water is shown after cooling; in hot exhaust it is vapour.

An ionic equation removes spectator ions that remain unchanged in solution. Split dissolved ionic compounds into ions; keep solids, gases, water and undissociated molecular substances intact. Cancel identical ions on both sides and check both atoms and total charge.

Worked exampleFind the reacting ions

Aqueous silver nitrate reacts with aqueous sodium chloride.

Try it first. Then show the worked stepsHide the worked steps
  1. AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq).
  2. Dissolved reactants provide Ag+, NO3-, Na+ and Cl-.
  3. Na+ and NO3- remain dissolved: cancel them.
Answer

Ag+(aq) + Cl-(aq) -> AgCl(s). Total charge is zero on both sides.

Check your understanding

To balance the formation of magnesium oxide, a student writes Mg + O2 -> MgO2. Is this acceptable?

  1. Yes. Both sides now have one magnesium atom and two oxygen atoms.
  2. No. It changes the formula of the product; write 2Mg + O2 -> 2MgO.
  3. No. Oxygen should be written as O, so the equation is Mg + O -> MgO.
Show the answer

No. It changes the formula of the product; write 2Mg + O2 -> 2MgO.

Balance by changing the numbers in front of formulae, never the subscripts. Magnesium oxide is MgO (Mg2+ and O2-), so the equation is 2Mg(s) + O2(g) -> 2MgO(s).

Check your understanding

Aqueous barium chloride is added to aqueous sodium sulfate, and a white precipitate forms. Which is the ionic equation?

  1. Na+(aq) + Cl-(aq) -> NaCl(s)
  2. Ba2+(aq) + SO42-(aq) -> BaSO4(s)
  3. BaCl2(aq) + Na2SO4(aq) -> BaSO4(s)
Show the answer

Ba2+(aq) + SO42-(aq) -> BaSO4(s)

Write only the ions that form the precipitate. Na+ and Cl- stay in solution unchanged, so they are spectator ions. The charges balance (2+ and 2- give 0).