Question

If the concentration of a `FeCl_3` solution is 2.5 M, what is the concentration of just chloride ions?

Answer 1

`"7.5 M"`

Explanation:

The idea here is that iron(III) chloride, `"FeCl"_3`, is soluble water, which implies that it dissociates completely to form iron(III) cations, `"Fe"^(3+)`, and chloride anions, `"Cl"^(-)`.

Now, notice that one formula unit of iron(III) chloride contains

• one iron(III) cation, `1 xx "Fe"^(3+)`
• three chloride anions, `3 xx "Cl"^(-)`

This means that when one mole of iron(III) chloride dissolves in water, you get

• one mole of iron(III) cations
• three moles of chloride anions

Therefore, you can say that the concentration of chloride anions in an aqueous solution of iron(III) chloride will be three times higher than the concentration of the salt itself.

In your case, you can say that the concentration of chloride anions will be

`color(green)(bar(ul(|color(white)(a/a)color(black)(["Cl"^(-)] = 3 xx "2.5 M" = "7.5 M")color(white)(a/a)|)))`