When solving any problems with the amounts of substances, you first need to write a balanced equation.
2H3PO4 --> P2O5 + 3H2O
This provides us with a way to make a connection between the amount of reactants and products based on moles of each. With just the information of the volume of phosphoric acid, there isn't enough information to be able to do that. If you have the molarity (mol H3PO4/L of soln), we could find moles of H3PO4 and be able to solve the problem.
For example, if the concentration was 5.0 M H3PO4, we could do the following
2.5 L * (5.0 mol/1 L) * (1 mol P2O5/2 mol H3PO4) * (141.9 g/mol P2O5)
As a result, we would have units of g of P2O5 and get an answer of 887 g. This answer will be different for a different concentration of H3PO4.