molecular weight
Summary
Molecular weight is the sum of the average atomic masses of all atoms in a molecular formula, in amu. It is numerically equal to molar mass in g/mol, so it converts grams to moles: n = m/M. Heavier gas molecules also effuse more slowly.
Detail
Meaning: Molecular weight is the sum of the atomic masses of every atom in a molecular formula, using the average values on the periodic table, which already account for natural isotope mixtures. It is measured in atomic mass units (amu) and is numerically equal to the molar mass in g/mol. That equality links one molecule to a countable lab quantity. For example, H₂O is about 18 amu, so one mole weighs about 18 g; CO₂ is about 44.
Equation: n = m/M, where n is the amount in moles (mol), m is the sample mass (kg; grams in practice), and M is the molar mass (kg/mol; g/mol in practice).
On the MCAT: It appears in stoichiometry and limiting reagent problems that convert grams to moles. It is used to find a molecular formula from an empirical formula and a given molar mass. It also appears in the gas density relation d = PM/RT. In Graham's law, effusion rate is proportional to 1/√M, so a lighter gas effuses faster than a heavier one at the same temperature.
Common confusion: Molecular weight (amu) and molar mass (g/mol) share a number but not a unit. Ionic compounds have no discrete molecules, so the term formula weight is used for them. "Weight" here means mass, not a gravitational force.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.