Here's how to determine the atomic mass of orthophosphoric acid (H₃PO₄):

1. Identify the elements and their atomic masses:

* Hydrogen (H): 1.008 amu

* Phosphorus (P): 30.974 amu

* Oxygen (O): 15.999 amu

2. Count the number of atoms of each element in the molecule:

* Hydrogen (H): 3 atoms

* Phosphorus (P): 1 atom

* Oxygen (O): 4 atoms

3. Multiply the atomic mass of each element by the number of atoms present:

* Hydrogen: 1.008 amu/atom * 3 atoms = 3.024 amu

* Phosphorus: 30.974 amu/atom * 1 atom = 30.974 amu

* Oxygen: 15.999 amu/atom * 4 atoms = 63.996 amu

4. Add the individual atomic masses together to get the total molecular mass:

* 3.024 amu + 30.974 amu + 63.996 amu = 97.994 amu

Therefore, the atomic mass of orthophosphoric acid (H₃PO₄) is 97.994 amu.