Question

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

Answer 1

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.