Question

a minute sample of AlCl3 is analyzed for chlorine.The analysis reveal that there are 12 chloride ions present in the sample. How many aluminium ions must be present in the sample

Answer 1

You can solve this by saying that there are 3 Cl for every 1 Al therefore a ratio of 1:3. If you have 12 Cl ions you can find how many Al ions there is
so (x)Al : 12 a unkown amount of Al ions gives you 12 Al ions from the ratio we know that we have 1 Al for 3 Cl therefore we can divide 12 by 3 to get the number of Al ions. The answer is 4 Al ions for 12 Cl ions.
Hope this helps :).