How do you write an ionic formula given an anion & cation? Use the phosphate anion as an example. What's the formula for the compound produced?
I think by using the phrase ionic formula you are talking about writing a chemical formula for a chemical composed on a cation and an anion. First of all, we always put the cation first and the anion second in the chemical formula. Second, you are going to have to memorize some ions and rules for determining them. I'm afraid there is no other way around this. Group 1 elements on the periodic table like sodium form the +1 cation. Group 2 elements like calcium form the +2 cation. Group 13 elements like Al form the +3 cation. Group 16 generally form the -2 anion. Group 17 (the halogens) form the -1 anion. The transition metals (groups 3-12) can form numerous different cations and you will simply have to memorize which ones form which ions (for example, copper can form the Cu^1+ and the Cu^2+ cations. We usually denote which ion of a transition metal we are referring to with a Roman numeral. So copper chloride could be CuCl or CuCl2 depending on which version of copper you are referring to. So use the Roman numerals to distinguish between the two: copper (I) chloride for CuCl and copper (II) chloride for CuCl2.
And then there are the polyatomic ions. These are ions that are composed of more than one element. They are usually anions and some examples are carbonate (CO3^2-, hydroxide (OH^1-), and phosphate (PO4^3-). These you will simply have to memorize as well.
Since you asked about the phosphate anion, let's look at a couple of examples. Potassium phosphate is one example. We know that the phosphate anion is PO4^3- and we know that potassium as a group 1 element forms the K^1+ cation. Since we have -3 for phosphate, then we need 3 potassium cations the balance the charges and have a neutral compound. So the chemical formula for potassium phosphate is K3PO4. But what about calcium phosphate? Calcium is a group 2 element and forms the +2 cation. So how do we balance this out in terms of charges? We look for the smallest number that has both 2 and 3 as a factor and that number is 6. If we have 3 of the calcium cations that will be +6. If we have 2 of the phosphate anions that will be -6, and thus the charges will be balanced for a neutral compound. So the chemical formula for calcium phosphate is Ca3(PO4)2. Please remember that you must use parentheses if you are referring to a group of atoms with a single subscript such as the two phosphates in the last example.