Reading a formula — answer key

1. FQ1 · Practice

Model answer. The molecular formula of a compound gives the number and type of different atoms in one molecule. For example, C₂H₄ says that one ethene molecule has two carbon atoms and four hydrogen atoms.

A full answer includes

• The number and type of different atoms in one molecule.

Also accept

• The number and type of different atoms in one molecule.

Watch for

• “The simplest whole number ratio of the atoms.” — That is the empirical formula (Supplement). The molecular formula counts the atoms in one molecule.
• “The formula shows how the atoms are arranged in space.” — A formula counts atoms; a model or a diagram shows their arrangement.

2. FQ2 · Practice

Model answer. The model has two carbon atoms (grey) and six hydrogen atoms (white), three on each carbon atom. Carbon comes first, then hydrogen, each with its count as a subscript: C₂H₆.

Checked automatically

• Carbon atoms in one molecule: 2 (exact value)
  • Common wrong answer 1 : There are two grey balls, joined to each other.
• Hydrogen atoms in one molecule: 6 (exact value)
  • Common wrong answer 3 : That is on one carbon atom. Count the hydrogen atoms on both.
  • Common wrong answer 4 : Count again: three on each carbon atom.

A full answer includes

• Carbon 2, hydrogen 6.
• Formula C₂H₆.

Also accept

• C 2, H 6; C₂H₆.

Watch for

• “Write CH₃, three hydrogen atoms on one carbon atom.” — CH₃ is half the molecule. The formula counts every atom in the whole molecule: C₂H₆.
• “Count the sticks as atoms.” — Sticks are bonds. Count only the balls.

3. FQ3 · Practice

Options as numbered on the pupil sheet: 1 — CH₄; 2 — H₂O; 3 — NaCl; 4 — CO₂; 5 — CaCO₃; 6 — NH₃.

Model answer. (1) Water is H₂O. (2) Carbon dioxide is CO₂: "di" means two oxygen atoms. (3) Methane is CH₄. (4) Ammonia is NH₃. (5) Sodium chloride is NaCl. (6) Calcium carbonate is CaCO₃.

A full answer includes

• 1 b; 2 d; 3 a; 4 f; 5 c; 6 e.

Also accept

• 1 H₂O; 2 CO₂; 3 CH₄; 4 NH₃; 5 NaCl; 6 CaCO₃.

Watch for

• “Carbon dioxide is CO.” — CO is carbon monoxide; "di" in dioxide means two oxygen atoms, CO₂.
• “Ammonia is NH₄.” — NH₄⁺ is the ammonium ion. The ammonia molecule is NH₃.

4. FQ4 · Practice

Model answer. The 2 after the bracket multiplies everything inside it: nitrogen 1 × 2 = 2 and oxygen 3 × 2 = 6. Calcium is outside the bracket: 1. That is 9 atoms in all.

Checked automatically

• Calcium atoms: 1 (exact value)
  • Common wrong answer 2 : Calcium is outside the bracket, so the 2 does not multiply it.
• Nitrogen atoms: 2 (exact value)
  • Common wrong answer 1 : The 2 after the bracket doubles the nitrogen too.
• Oxygen atoms: 6 (exact value)
  • Common wrong answer 3 : There are 3 in each NO₃, and there are two of them.
  • Common wrong answer 5 : Multiply, don't add: 3 × 2 = 6.

A full answer includes

• Ca 1, N 2, O 6.

Also accept

• Ca 1, N 2, O 6.

Watch for

• “The number after a bracket multiplies only the last symbol.” — The number after a bracket multiplies everything inside the bracket.
• “Ca(NO₃)₂ has 3 oxygen atoms.” — There are 3 in each nitrate group, and the bracket doubles it: 6.

5. FQ5 · Practice

Model answer. The box holds three identical molecules. Each has two oxygen atoms joined to each other and one hydrogen atom on each oxygen atom: two hydrogen atoms and two oxygen atoms. The formula describes one molecule, so it is H₂O₂, not H₆O₆.

A full answer includes

• H₂O₂: each molecule has 2 hydrogen and 2 oxygen atoms.

Also accept

• H₂O₂.

Watch for

• “Count every atom in the diagram and write that as the formula.” — A formula describes one molecule. Count the atoms in one molecule only.
• “Hydrogen peroxide is HO.” — HO is the simplest ratio (the empirical formula). The molecule itself has two of each atom.

6. FQ6 · Practice

Model answer. Oxygen and nitrogen are molecules of two atoms: O₂ and N₂. Helium is a noble gas of single atoms: He. Magnesium is a metal, a giant structure, so it is written as its symbol: Mg. Chlorine is Cl₂, not Cl, and sodium is a metal, so it is Na, not Na₂.

A full answer includes

• O₂, N₂, He and Mg.

Also accept

• O₂, N₂, He, Mg.

Watch for

• “Oxygen, hydrogen, nitrogen and chlorine are written as single atoms, O, H, N and Cl.” — These elements exist as molecules of two atoms, so they are written O₂, H₂, N₂ and Cl₂.
• “Every element has a 2.” — Only some non-metals are molecules of two atoms. Metals and noble gases are written as the symbol alone.

7. FQ7 · Practice

Model answer. Both 2 and 6 divide by 2, giving 1 carbon to 3 hydrogen: the empirical formula is CH₃. No larger number divides both, so CH₃ is the simplest whole number ratio.

A full answer includes

• CH₃.

Also accept

• CH₃.

Watch for

• “The empirical formula is always the molecular formula halved.” — Divide by the largest number that goes into every subscript; it may be 2, 3, 6 or none at all.
• “The empirical formula of ethane is C₂H₆.” — C₂H₆ is the molecular formula. Both numbers divide by 2, so the simplest ratio is CH₃.

8. FQ8 · Practice

Options as numbered on the pupil sheet: 1 — C₃H₈; 2 — HO; 3 — NO₂; 4 — C₂H₅; 5 — CH₂.

Model answer. (1) H₂O₂ divides by 2: HO. (2) C₄H₁₀ divides by 2: C₂H₅. (3) C₃H₈: nothing larger than 1 divides both 3 and 8, so it stays C₃H₈. (4) N₂O₄ divides by 2: NO₂.

A full answer includes

• 1 b; 2 d; 3 a; 4 c.

Also accept

• 1 HO; 2 C₂H₅; 3 C₃H₈; 4 NO₂.

Watch for

• “The empirical formula is always the molecular formula halved.” — Divide by the largest number that goes into every subscript; it may be 2, 3, 6 or none at all.
• “C₄H₁₀ simplifies to CH₂.” — Divide both numbers by the same number: 4 ÷ 2 = 2 and 10 ÷ 2 = 5, C₂H₅.

9. FQ9 · Practice

Options as numbered on the pupil sheet: 1 — MgCl₂; 2 — Ca(OH)₂; 3 — NaCl; 4 — Al₂O₃; 5 — MgCl.

Model answer. (1) Na⁺ and Cl⁻: +1 and −1 cancel, NaCl. (2) Mg²⁺ and Cl⁻: +2 needs two −1, MgCl₂. (3) Al³⁺ and O²⁻: two Al³⁺ (+6) and three O²⁻ (−6), Al₂O₃. (4) Ca²⁺ and OH⁻: +2 needs two OH⁻, kept in a bracket, Ca(OH)₂.

A full answer includes

• 1 c; 2 a; 3 d; 4 b.

Also accept

• 1 NaCl; 2 MgCl₂; 3 Al₂O₃; 4 Ca(OH)₂.

Watch for

• “One positive ion always joins one negative ion.” — Take just enough ions for the charges to add up to zero: Mg²⁺ needs two Cl⁻.
• “Write calcium hydroxide as CaOH₂.” — Without a bracket, the 2 doubles only the hydrogen. Two whole OH⁻ ions are written (OH)₂.

10. FQ10 · Practice

Model answer. Inside the box there are 4 calcium ions and 8 fluoride ions, a ratio of 1 : 2. The charges agree: one Ca²⁺ (+2) is balanced by two F⁻ (−2). The formula is CaF₂.

A full answer includes

• CaF₂: 4 calcium ions to 8 fluoride ions is 1 : 2.

Also accept

• CaF₂.

Watch for

• “Write the numbers you counted, Ca₄F₈.” — An ionic formula gives the simplest ratio of the ions: 4 : 8 is 1 : 2, CaF₂.
• “One positive ion always joins one negative ion.” — Take just enough ions for the charges to add up to zero: Mg²⁺ needs two Cl⁻.

11. FT1 · Apply

Model answer. (a) Chlorine exists as molecules of two atoms: Cl₂. The molecular formula gives the number and type of atoms in one molecule: two chlorine atoms. (b) Aluminium 2; the 3 after the bracket multiplies everything inside it, so sulfur 1 × 3 = 3 and oxygen 4 × 3 = 12. (c) One atom each of hydrogen, oxygen and chlorine: HOCl, written in the order the atoms are joined (HClO is also accepted). (d) The smallest total both charges can make is 6: two Al³⁺ give +6 and three SO₄²⁻ give −6, so the compound has no overall charge. Sulfate is a group that stays together, so three of them are written (SO₄)₃. (e) 2 and 4 both divide by 2: NH₂.

A full answer includes

• (a) Cl₂: one molecule has two chlorine atoms.
• (b) Al 2, S 3, O 12.
• (c) HOCl, one atom of each (also written HClO).
• (d) Two Al³⁺ give +6 and three SO₄²⁻ give −6, so the charges cancel: Al₂(SO₄)₃.
• (e) NH₂.

Also accept

• (a) Cl₂; two chlorine atoms in one molecule. (b) Al 2, S 3, O 12. (c) HOCl. (d) 2 × (+3) = +6 and 3 × (−2) = −6. (e) NH₂.

Watch for

• “Oxygen, hydrogen, nitrogen and chlorine are written as single atoms, O, H, N and Cl.” — These elements exist as molecules of two atoms, so they are written O₂, H₂, N₂ and Cl₂.
• “The number after a bracket multiplies only the last symbol.” — The number after a bracket multiplies everything inside the bracket.
• “One positive ion always joins one negative ion.” — Take just enough ions for the charges to add up to zero: Mg²⁺ needs two Cl⁻.