The mass of a molecule is expressed in atomic mass units (amu) or Daltons (Da), which are approximately equal. One amu is defined as 1/12 of the mass of a carbon-12 atom.

A water molecule consists of two hydrogen atoms and one oxygen atom. The atomic mass of hydrogen is approximately 1 amu, and the atomic mass of oxygen is approximately 16 amu.

Therefore, the mass of a water molecule is approximately 18 amu.

In scientific notation, the mass of a water molecule is approximately 2.99 x 10^-23 gram.