Chemguide: Core Chemistry 14 - 16

Answers to Set 2 of the formulae problems

sodium bromide: NaBr

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

To balance the charges, you need one of each.

copper(II) sulfate: CuSO₄

The Roman numeral (II) in the name tells you that copper ion has a 2+ charge; sulfate is an ion whose formula and charge you have to remember. Sulfate ions are SO₄^2-. (If you confused sulfide and sulfate, remember to look carefully at the endings of the words.)

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

If you put brackets around the sulfate group, that is wrong.

barium hydroxide: Ba(OH)₂

Barium is in Group 2 and so its ion has a 2+ charge; hydroxide is an ion whose formula and charge you have to remember. Hydroxide ions are OH^-.

To balance the charges, you need two hydroxide ions for each barium ion.

(If you forgot to write the brackets around the hydroxide group, your answer is wrong. If you wrote BaOH₂, that means 1 barium, 1 oxygen and 2 hydrogens. The brackets show that you have both 2 oxygens and 2 hydrogens.)

nickel chloride: NiCl₂

The Roman numeral (II) in the name tells you that nickel ion has a 2+ charge; chlorine is in Group 7 and so its ion has a 1- charge.

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

lithium nitrate: LiNO₃

Lithium is in Group 1 and so its ion has a 1+ charge; nitrate is an ion whose formula and charge you have to remember. Nitrate ions are NO₃^-. (If you confused nitrate with nitride, remember to look carefully at the endings of the words.)

To balance the charges, you need one of each.

If you put brackets around the nitrate group, that is wrong.

Return to the problems . . .