The mass number of a molecule is the sum of the number of protons and neutrons in its nucleus. For H2O, we have:

- 2 Hydrogen atoms, each with 1 proton and 1 neutron, for a total of 2 protons and 2 neutrons.

- 1 Oxygen atom, with 8 protons and 8 neutrons.

Therefore, the mass number of H2O is 2 + 2 + 8 + 8 = 20.