How is molecular weight calculated?

Molecular weight, also referred to as molecular mass, is determined by summing the atomic weights of all the atoms in a molecule. Each element has a specific atomic weight listed on the periodic table, which is typically given in atomic mass units (amu). When calculating the molecular weight, you take into consideration the number of each type of atom present in the molecule and multiply their atomic weights by these quantities.

For instance, in water (H₂O), you would sum the atomic weights of two hydrogen atoms and one oxygen atom to find the total molecular weight. This process accurately reflects the mass of the entire molecule based on the contributing parts from the periodic table.

The other choices do not accurately define how molecular weight is calculated. For example, averaging the atomic masses of all elements does not consider the specific composition of the molecule. Similarly, focusing on the weight of the heaviest atom or the heaviest isotopes does not account for all the atoms in the molecule, leading to an incomplete understanding of molecular weight.