Calculate the mole fraction of phosphoric acid (H3PO4) in a 25.4% (by mass) aqueous solution.

First of all, the problem is giving us the amount of the sample by mass percentage, then let's consider we have 100 g the total mass of the sample.

number of moles of H₃PO₄ = mass / molar mass

number of moles of H₃PO₄ = 25.4 / 97.99 = 0.26 moles

number of moles of H₂O = mass / molar mass

number of moles of H₂O = 74.6 / 18 = 4.14 moles

the mole fraction of phosphoric acid (H₃PO₄) = moles of phosphoric acid / the total moles of the sample

the mole fraction of phosphoric acid (H₃PO₄) = (0.26) / (0.26 + 4.14) = 0.059

Note that the mole fraction always have no unit because its just mole ratio.