To determine the molecular weight of a molecule of water (H2O), you need to add the atomic weights of the hydrogen and oxygen atoms that make up the molecule. Here's how you would calculate it:

1. Look up the atomic weights of hydrogen (H) and oxygen (O) from the periodic table:

- Hydrogen (H): 1.008 atomic mass units (amu)

- Oxygen (O): 15.999 amu

2. Multiply the atomic weight of each element by the number of atoms in the molecule:

- Hydrogen: 2 atoms of H × 1.008 amu/atom = 2.016 amu

- Oxygen: 1 atom of O × 15.999 amu/atom = 15.999 amu

3. Add up the atomic weights of all the atoms in the molecule:

- 2.016 amu (H) + 15.999 amu (O) = 18.015 amu

Therefore, the molecular weight of a water molecule (H2O) is approximately 18.015 atomic mass units.