Chemguide: Core Chemistry 14 - 16

Answers to Set 4 of the formulae problems

potassium carbonate: K₂CO₃

Potassium is in Group 1 and so its ion has a 1+ charge; carbonate is an ion whose formula and charge you have to remember. Carbonate ions are CO₃^2-.

To balance the charges, you need two potassium ions for each carbonate ion.

aluminium oxide: Al₂O₃

Aluminium is in Group 3 and so its ion has a 3+ charge; oxygen is in Group 6 and so its ion has a 2- charge.

To balance the charges, you need to have a total of 6+ and 6- charges. You choose the smallest number which both the 2 and 3 will divide into - in this case, 6.

silver iodide: AgI

Silver is an ion whose charge you need to remember; iodine is in Group 7 and so an iodide ion has a 1- charge.

To balance the charges, you need one of each.

lead(II) nitrate: Pb(NO₃)₂

The Roman numeral (II) in the name tells you that the lead ion has a 2+ charge; nitrate ions are NO₃^-.

To balance the charges, you need 2 nitrate ions for each lead(II) ion.

If you forgot the brackets, it is wrong.

aluminium hydroxide: Al(OH)₃

Aluminium is in Group 3 and so its ion has a 3+ charge; hydroxide is an ion you wil have to learn. Hydroxide ions are OH^-.

To balance the charges, you need three hydroxide ions for each aluminium ion.

If you forgot the brackets, it is wrong.

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