The molecular weight of H2O (water) is 18 g/mol.

So, if the molar mass of oxygen (O) is 16 g/mol and the molar mass of hydrogen (H) is 1 g/mol, then the molar mass of H2O (water) can be calculated as:

2(H) + 1(O) = 2(1) + 16 = 18 g/mol

Therefore, the molar mass of H2O (water) is 18 g/mol.