What is molecular weight?

What is molecular weight?

• A. the sum of the atomic weights of the elements in a compound, expressed in grams per mole
• B. the weight of a single molecule in grams
• C. the weight of a compound per liter
• D. the ratio of its mass to its volume

Answer: (A)

Molecular weight is the total mass contributed by all atoms in one molecule. You add up the atomic weights of each element in the molecule (multiplied by how many of each atom are present), and that sum is the molecular weight. When you express it in grams per mole, you’re describing how heavy one mole of those molecules would be—the molar mass. So the idea is the sum of the atoms’ weights, with the unit grams per mole providing a mass per amount. For example, a water molecule has two hydrogens and one oxygen, giving about 2×1.01 + 16.00 ≈ 18.02 g/mol as its molar mass (and about 18.02 amu as its molecular weight in atomic mass units). The other options don’t fit: the mass of a single molecule in grams is not the standard expression; density is mass per volume; and mass per volume per liter is a concentration, not molecular weight.

Molecular weight is the total mass contributed by all atoms in one molecule. You add up the atomic weights of each element in the molecule (multiplied by how many of each atom are present), and that sum is the molecular weight. When you express it in grams per mole, you’re describing how heavy one mole of those molecules would be—the molar mass. So the idea is the sum of the atoms’ weights, with the unit grams per mole providing a mass per amount.

For example, a water molecule has two hydrogens and one oxygen, giving about 2×1.01 + 16.00 ≈ 18.02 g/mol as its molar mass (and about 18.02 amu as its molecular weight in atomic mass units). The other options don’t fit: the mass of a single molecule in grams is not the standard expression; density is mass per volume; and mass per volume per liter is a concentration, not molecular weight.