Chemguide: Core Chemistry 14 - 16

Answers to Set 1 of the formulae problems

potassium iodide: KI

Potassium is in Group 1 and so its ion has a 1+ charge; iodine is in Group 7 and so its ion has a 1- charge.

To balance the charges, you need one of each.

iron(II) sulfide: FeS

The Roman numeral (II) in the name tells you that iron ion has a 2+ charge; sulfur is in Group 6 and so its ion has a 2- charge.

To balance the charges, you again need one of each.

calcium chloride: CaCl₂

Calcium is in Group 2 and so its ion has a 2+ charge; chlorine is in Group 7 and so its ion has a 1- charge.

To balance the charges, you need two chloride ions for each calcium ion.

magnesium carbonate: MgCO₃

Magnesium is in Group 2 and so its ion has a 2+ charge; carbonate is an ion whose formula and charge you have to remember. Carbonate ions are CO₃^2-.

To balance the charges, you need one of each.

You should not have written any brackets around the carbonate part.

lithium oxide: Li₂O

Lithium is in Group 1 and so its ion has a 1+ charge; oxygen is in Group 6 and so its ion has a 2- charge.

To balance the charges, you need two lithium ions for each oxide ion.

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