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Science

Ionic compounds

Formulae of ionic compounds

Sea water contains a number of dissolved salts. When these salts dissolve in water, the ions separate. Sea water therefore contains a mixture of ions. The most common ions in sea water are shown in the table:

 

Name of ionFormula and charge
ChlorideCl
SodiumNa+
SulfateSO42−
MagnesiumMg2+
CalciumCa2+
PotassiumK+
CarbonateCO32−
BromideBr

It is possible to deduce from the table which salts dissolved to form the mixture of ions:

  • When sodium chloride dissolves it forms sodium ions and chloride ions

  • When magnesium chloride dissolves it forms magnesium and chloride ions

  • When magnesium sulfate dissolves if forms magnesium ions and sulfate ions

  • When potassium chloride dissolves it forms potassium ions and chloride ions

  • When potassium bromide dissolves it forms potassium ions and bromide ions

You can use the charge on the ions shown in the table to work out the formulae of the ionic compounds.

Sodium ions each have a single positive charge. Chloride ions each have a single negative charge. For the charges to cancel out in the neutral salt sodium chloride, they must be in a ration of 1:1. So the formula of sodium chloride is NaCl.

Magnesium ions each have two positive charges. Chloride ions each have a single negative charge. For the charges to cancel out in the neutral salt magnesium chloride, they must be in a ration of 1:2. So the formula of magnesium chloride is MgCl2.

Read on if you are taking the higher paper.

Back to Ionic compounds and analysis index

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