How do you calculate the number of ions in a solution?

Find the molar concentration then look at the formula to work out the concentration of each ion.

Lets look at an example: "How many sodium ions are there in a solution of sodium chloride of concentration #58.5 g.dm^(-3) ?# "

We need to convert this into #mol.dm^(-3)# . To do this we add up the #A_r# values to get the relative formula mass. For NaCl this will be 23 +35.5 = 58.5. So 1 mole of NaCl weighs 58.5g.

So now we need to convert grams into moles by dividing mass in grams by the mass of 1 mole.

So the number of moles of NaCl = 58.5/58.5 = 1 mole

So the concentration of NaCl is #1 mol.dm^(-3)#

This means that in #1dm^3# of solution there must be 1 mole of sodium ions.

The number of particles in 1 mole is given by the Avogadro Constant which is equal to #6.02x10^(23) mol^(-1)#. We usually give this the symbol L.

So the solution in question contains L sodium ions.

Check the stoichiometry of the formula. If you have #1dm^3# of a 1 molar solution of sodium sulfate #Na_2SO_4# then this would be 2 molar with respect to sodium ions so would contain 2L sodium ions.