Final answer:
The relative formula mass of H₂O (water) is 18.02 g/mol.
Step-by-step explanation:
The relative formula mass of a compound is calculated by summing up the atomic masses of all the atoms in its formula. For example, the relative formula mass of H₂O (water) is calculated by adding the atomic masses of two hydrogen atoms (2x1.01) and one oxygen atom (1x16.00), resulting in a relative formula mass of 18.02 g/mol. This means that one mole of water molecules weighs 18.02 grams.