What is the molecular weight of water (H2O)?

To determine the molecular weight of water (H2O), we need to look at the atomic masses of the constituent elements, hydrogen and oxygen.

Hydrogen has an atomic mass of approximately 1 g/mol, and since there are two hydrogen atoms in a water molecule, the contribution from hydrogen is 2 x 1 g/mol = 2 g/mol.

Oxygen, on the other hand, has an atomic mass of about 16 g/mol. Therefore, the total contributions to the molecular weight of water combines both the hydrogen and oxygen contributions:

2 g/mol (from hydrogen) + 16 g/mol (from oxygen) = 18 g/mol.

This calculation shows that the molecular weight of water is 18 g/mol, confirming that the correct answer is indeed C. Understanding this calculation reinforces the importance of knowing the atomic masses of elements when determining the molecular weight of compounds.