Which of the following represents the mass of water?

The mass of water is best represented by 18 g/mol. This value corresponds to the molecular weight of water, which consists of two hydrogen atoms and one oxygen atom. The atomic mass of hydrogen is approximately 1 g/mol, and that of oxygen is about 16 g/mol. Therefore, when calculated, the molecular weight of water (H₂O) is:

[ 2 \times 1 , \text{g/mol} + 16 , \text{g/mol} = 18 , \text{g/mol} ]

This molecular weight is a standard value used in chemistry to describe the mass of one mole of water.

The other options do not accurately reflect the molecular weight of water. For instance, 65 kg/mol is significantly higher than the actual weight and might refer to a larger molecular entity. The value 100 billion g/mol is astronomically large and is not relevant to a simple molecule like water. Lastly, 507 g/mol is also much greater than the true molecular weight of water and may suggest a complex molecule or synthetic compound not related to water.