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Leaving Cert Physics & Chemistry: Formulas & equations

How often Formulas & equations comes up on the Physics & Chemistry papers, every year it was asked, and questions to try.

HL Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

OL Asked on 9 of the last 9 Ordinary Level papers, most recently in 2025. banker

Formulas & equations, Higher Level(7 marks)

Quick ones on Formulas & equations.

(a)Formula of aluminium oxide?
  1. AlO
  2. Al2O3
  3. Al3O2
(b)Percentage by mass of nitrogen in NH4NO3? (N = 14, H = 1, O = 16)
  1. 60%
  2. 17.5%
  3. 35%
(c)Formula of calcium nitrate?
  1. Ca(NO3)2
  2. Ca2NO3
  3. CaNO3
Show the answers

(a) Al2O3

(b) 35%

(c) Ca(NO3)2

Your turn: Higher Level questions on Formulas & equations.

Higher Level

Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

Every paper, year by year

YearWhere it came up
2025Q7(d), Q7(e)
2024Q7(h), Q7(k)
2023Q7(e), Q7(g), Q7(i)
2022Q7(h)
2021Q7(i), Q7(n)
2019Q7(g)
2018Q7(d), Q7(g), Q7(k)
2017Q7(k)
2016Q7(g), Q7(l), Q7(n)

Links open the State Examinations Commission’s paper for that year.

More Formulas & equations questions

Formulas & equations, 2 marks

Formula of iron(III) chloride?

  1. FeCl2
  2. Fe3Cl
  3. FeCl3
Show the answer

FeCl3

The (III) means iron has valency 3 here (Fe3+). Chlorine has valency 1, so three Cl- ions are needed. FeCl2 is iron(II) chloride.

Formulas & equations, 3 marks

A hydrocarbon is 80% carbon and 20% hydrogen by mass. Empirical formula? (C = 12, H = 1)

  1. C4H
  2. CH3
  3. CH4
Show the answer

CH3

Moles: C = 80/12 = 6.67, H = 20/1 = 20. Divide by the smaller: C 1, H 3, giving CH3. (The molecule is ethane, C2H6.)

Formulas & equations, 2 marks

Formula of magnesium hydroxide?

  1. Mg(OH)2
  2. MgOH
  3. Mg2OH
Show the answer

Mg(OH)2

Mg has valency 2 and hydroxide (OH-) has valency 1, so two OH groups are needed. The bracket shows the whole OH group is doubled.

Ordinary Level

Asked on 9 of the last 9 Ordinary Level papers, most recently in 2025. banker

Every paper, year by year

YearWhere it came up
2025Q7(i), Q7(m), Q7(n)
2024Q7(h), Q7(l)
2023Q7(h), Q7(l)
2022Q7(f), Q7(k)
2021Q7(i), Q7(m)
2019Q7(i), Q7(k)
2018Q7(i), Q7(m)
2017Q7(h)
2016Q7(h), Q7(i)

Links open the State Examinations Commission’s paper for that year.

More Formulas & equations questions

Formulas & equations, 2 marks

Formula of calcium chloride (valency of Ca = 2, Cl = 1)?

  1. CaCl
  2. Ca2Cl
  3. CaCl2
Show the answer

CaCl2

Ca2+ needs two Cl- ions to balance its charge. Swap the valencies: Ca gets 1, Cl gets 2.

Formulas & equations, 3 marks

Formula of aluminium oxide (valency of Al = 3, O = 2)?

  1. Al3O2
  2. Al2O3
  3. AlO
Show the answer

Al2O3

Swap the valencies: Al gets 2 and O gets 3. Check: 2 × 3 = 6 positive and 3 × 2 = 6 negative, so the charges balance.

Formulas & equations, 2 marks

Balanced equation for hydrogen burning in oxygen?

  1. 2H2 + O2 → 2H2O
  2. H2 + O2 → H2O
  3. H2 + O2 → 2H2O
Show the answer

2H2 + O2 → 2H2O

Count atoms on each side. Left: 4 H and 2 O. Right: 2H2O has 4 H and 2 O. Never change a formula to balance; change the numbers in front.

Other Physics & Chemistry topics

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