[MattyW]

I dont understand why the 3 and 4 are in h3po4 (phosphoric acid) or how they got there. ive googled almost everything i could think of and all i get is the end result. id like to understand how to work out how to get this formula. any help would be greatly appreciated thanks

- Matty

[AWK]

This is just information on contents of individual molecula of phosphoric(V) acid nothing to undersand.

[DevaDevil]

Matty, one of the things in chemistry you should learn is a list of common ions. Phosphate is one of those: PO₄^3-. You will just have to remember them

Now because the ion is 3- charged, it will be able to bind with 3 protons (H⁺), to form the charge-neutral phosphoric acid; H₃PO₄.

[Cavillus]

Well, I generally agree with AWK and DevaDevil, but rather than merely memorizing the most common ions it would be more useful to understand the logic behind molecules building.

So, I'll show you a little example of this logic with the molecule you wrote, phosphoric acid (H₃PO₄).
Of course, you need to know a little bit of chemistry. If you aren't at this level, write this stuff down and check it again when you will be able to completely understand it.
As you certainly know, Phosphorus lies in the 5th group, so he has 5 electrons in his outer shell.
In order to get to the oxyacid written above, you need to react the corresponding anhydride with water.
The anhydride is a binary compound of the given element and oxygen. In this kind of compounds, oxygen's valency is always 2. Phosphorus, instead, has several plausible valencies. As the name suggests, P valency for this compound is the maximum available, 5.
The minimum formula for the phosphoric anhydride, so, is P₂O₅ (it has to be precised that the real formula is P₄O₁₀, but these details aren't fundamental for the correct comprehension of the method).
We can, then write the reaction with water:

P₂O₅ + 3 H₂O  2 H₃PO₄

This specific acid has even another little subtlety, because the formula of the acid obtained varies with the number of the water molecules added:
1 molecule: metaphosphoric acid (HPO₃);
2 molecules: pyrophosphoric acid (H₄P₂O₇);
3 molecules: orthophosphoric acid (H₃PO₄);

Anyway, when you have sufficiently metabolized this rules you must learn the most common anions occuring in chemistry courses, as other people rightly state.