To find the molar mass of water $H2O$, we first need to find the molar mass of hydrogen $H$ and oxygen $O$ separately.

The molar mass of hydrogen $H$ is 1 g/mol and there are 2 atoms of hydrogen in water $H2O$, so the total molar mass of hydrogen in water is:
1 g/mol * 2 = 2 g/mol

The molar mass of oxygen $O$ is 16 g/mol.

Adding the molar masses of hydrogen and oxygen gives us the molar mass of water:
2 g/mol $H$ + 16 g/mol $O$ = 18 g/mol

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