Question

When 0.076 moles of iron(III) chloride are dis- solved in enough water to make 690 milliliters of solution, what is the molar concentration of chloride ions?

Answer 1

`"0.330 M"`.

Explanation:

Molar concentration, or molarity, just refers to how many moles of the solute there is per `1` litre of solution:

`"x moles"/"1 L"`

We have `0.076` moles of iron(III) chloride, or `FeCl_3`, in `"690 mL"` of solution:

`"0.076 moles"/"690 mL" = "0.076 moles"/"0.690 L"`

We can divide `0.076` moles by `0.690` to find the molarity of `FeCl_3`:

`"0.076 moles/0.690"/"0.690 L/0.690" = "0.110 moles"/"1L" = "0.110 M"`

From the formula of `FeCl_3`, we know that, for every `1` mole of the entire compound, there are `3` moles of chloride ions.
That means that for every `0.110` moles of `FeCl_3`, there are `0.110xx3=0.330` moles of chloride ions.

Therefore, its molarity is:
`"0.330 moles"/"1L" = "0.330 M"`