The answer to this question would be:5 mol
To answer this question, you need to know the chemical equation of mg3(po4)2 when forming an ion. The equation should be: mg3(po4)2= 3Mg(2+) +2PO4(3-)
That means for 1 mol of mg3(po4)2 there will be 3 magnesium(cation) ion and 2 phosphates(anion) ion, the result would be 5 mol of ion.
