Phosphate (PO4)3– is a polyatomic ion with a 3– charge.

Question

What is the chemical formula for the ionic compound lithium phosphate?

Li3PO4
LiPO4
Li(PO4)3
Li2PO4

would it be Li(PO4)3?

Answer 1

Nope. Li is +1. PO4 is -3. All compounds have a net charge of zero so lithium phosphate must be zero. If you have Li at +1 and PO4 at -3 then there must be 3 Li to make +3 to balance the -3 for PO4. There is an easy way of doing this. Write Li PO4 below with the charges above them like this (but it's tough on spacing with this html) so I will use ..........to help space.
..........+1..-3
............Li.y.(PO4)x
Now draw a diagonal line from the +1 to the place I've marked with an x and another diagonal line from the -3 to the place I've marked with a y. That tells you y is 3 (always ignore the signs) and the x is 1. When it is one you don't need to write it. So the formula is Li3PO4.
What about calcium phosphate, Ca and PO4. It looks this way
Ca3(PO4)2. If we did it up brown with real subscripts it would look this way.\
Ca₃(PO₄)₂.

NNow draw line

Wr

Answer 2

TL;DR
The answer is Li3PO4.