Question

What is the formula for `Ba_3(PO_4)_2`?

Answer 1

Have you not just quoted it?

Explanation:

I suspect that you would have to work hard to get water-free barium phosphate.... You are more likely to isolate `BaHPO_4`, i.e. the salt of `Ba^(2+)` and biphosphate ion, `HPO_4^(-)`.