What is molecular mass?

Molecular mass refers to the sum of the masses of all the atoms in a molecule, calculated based on the number and types of atoms present. It is expressed in atomic mass units (amu) and is determined by adding up the atomic masses of each element in the molecular formula. For instance, in water (H2O), the molecular mass would be calculated by adding the masses of two hydrogen atoms and one oxygen atom.

This concept is essential in chemistry because it allows chemists to understand the proportions of substances involved in chemical reactions, enabling accurate calculations in stoichiometry. In contrast, the other options discuss different concepts that do not align with the precise definition of molecular mass. For example, the total weight of a compound including water suggests a focus on hydration and solvent effects, while the mass of one mole of a gas refers to molar mass. Additionally, discussing the average mass of an element's isotopes pertains to atomic mass rather than molecular mass.