Look up molar masses for the elements that comprise H₃PO₄:
H = 1.008 g/mol
P = 30.97 g/mol
O = 15.99 g/mol
The molar mass of H₃PO₄ is a weighted sum of these (3 hydrogen + 1 phosphorus + 4 oxygen) that comes out to about 97.99 g/mol.
Then
(18 g H₃PO₄) × (1/97.99 mol/g) ≈ 0.1837 mol
