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.  

The mole fraction of phosphoric acid (H₃PO₄) in a 25.4% (by mass) aqueous solution is 0.059

Let the mass of the solution be 100 g.

Thus, the mass of 25.4% of H₃PO₄ = 25.4 g

Mass of H₃PO₄ = 25.4 g

Molar mass of H₃PO₄ = (3×1) + 31 + (16×4) = 98 g/mol

Mole = mass / molar mass

Mole of H₃PO₄ = 25.4 / 98

Mass of H₂O = 100 – 25.4

Mass of H₂O = 74.6 g

Molar mass of H₂O = (2×1) + 16 = 18 g/mol

Mole = mass / molar mass

Mole of H₂O = 74.6 / 18

Mole of H₃PO₄ = 0.259 mole

Mole of H₂O = 4.144 moles

Total mole = 0.259 + 4.144 = 4.403

Mole fraction = mole / total mole

Mole fraction of H₃PO₄ = 0.259 / 4.403

Thus, the mole fraction of H₃PO₄ is 0.059

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