Determining ionic formulae | Exit Quiz | Oak National Academy

Match the following key terms to the correct definition.

• molecular formula

• Simplest whole number ratio of atoms of each element in a compound.

• polyatomic ions

• Name ending for a compound with 3 or more elements, one being oxygen.

• empirical formula

• Name ending for a compound with a metal and singular non–metal.

• –ide compounds

• Ions made of more than one type of atom.

• –ate compounds

• Shows the actual number of atoms of each element in a compound.

Answer explanation

The formulae for ionic compounds are empirical, rather than molecular, as there is no definite number of each atom in the structure. Ions can contain more than one type of atom e.g. the sulfate ion contains sulfure and oxygen.

The molecular formula for hydrogen peroxide is H₂O₂. What is its empirical formula?

Answer explanation

The ratio of hydrogen atoms to oxygen atoms in the molecular formula is 2 : 2. This can be simplified to 1 : 1, so the empirical formula is HO.

Which of the following are examples of ionic compounds with names ending with –ide?

• NaCl

• MgSO₄

• H₂O

• CaO

Answer explanation

Magnesium sulfate, MgSO₄, is an ionic compound containing three elements, including oxygen. Water, H₂O, is a simple covalent compound. Sodium chloride, and calcium oxide are both ionic compounds which only contain one metal and one non–metal ion.

Which of the following are examples of polyatomic ions?

• Na⁺

• SO₄²⁻

• Cl⁻

• Cu(NO₃)₂

• CO₃⁻

Answer explanation

The sulfate ion (SO₄²⁻) and the carbonate ion (CO₃⁻) are polyatomic.

The image shows the ionic compound lead(II) chloride. What is the empirical formula for lead(II) chloride?

• PbCl

• PbCl₂

• Pb₂Cl

Answer explanation

The ratio of lead ions to chloride ions is 1 : 2, this means the empirical formula is PbCl₂.

The ammonium ion has the formula: NH₄⁺. Sulfate ions have the formula: SO₄²⁻. What is the formula for ammonium sulfate?

• NH₄⁺SO₄²⁻

• NH₄SO₄

• NH₄(SO₄)₂

• (NH₄)₂SO₄

Answer explanation

The ammonium has a 1+ charge and the sulfate has a 2– charge. The total charge must balance out so that the overall charge is neutral. This means there must be two ammonium ions and one sulfate ion in the formula: (NH₄)₂SO₄.