# Given a selection of anions, phosphate, sulfate, nitrate, how do we form salts with cations?

Dec 6, 2016

You have to know the charge on the constituent ion, and then these combine in such a way to form a NEUTRAL salt.

#### Explanation:

So we we've got a selection of anions, say $P {O}_{4}^{3 -}$, $S {O}_{4}^{2 -}$, and ${P}^{3 -}$. You simply have to know these species as common ions.

And now, these make music with a selection of common CATIONS, $C {a}^{2 +}$, $F {e}^{3 +}$, $N {a}^{+}$. Now all matter is electrically neutral, so when these ions combine, they do so in a way as to make the overall electric charge $0$, i.e. the salt is NEUTRAL. Are we clear on this? If no, voice your objections, and we will have another go.

So with phosphate ion, $P {O}_{4}^{3 -}$, we would form $C {a}_{3} {\left(P {O}_{4}\right)}_{2}$, $F e \left(P {O}_{4}\right)$, and $N {a}_{3} \left(P {O}_{4}\right)$. All I have done here is to cross-multiply to give an ionic salt in which negative and positive charges are EQUAL such that the charge on the SALT is neutral.

And with sulfate, we get $C a S {O}_{4} , F {e}_{2} {\left(S {O}_{4}\right)}_{3} , N {a}_{2} S {O}_{4}$.

You try this, and make neutral salts with the phosphide anion, ${P}^{3 -}$. I grant that this appears abstract, but all we are doing is making sure that the ionic solid is NEUTRAL. Again, come back for another serve if you don't see it.