Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Molecular Mass
Molecular Mass
The molecular mass of a substance is the mass of a single molecule of that substance, again expressed relative to the mass of one carbon-12 atom (i.e. on the same u scale used for atomic mass) rather than in grams directly. It is worked out as the sum of the average atomic masses of every element present in the molecule, where each element's average atomic mass is first multiplied by however many atoms of that element the molecule's formula contains, and all of these products are then added together. Carbon dioxide (CO2) is a worked example in the text: its molecular mass is 1 x (average atomic mass of carbon) + 2 x (average atomic mass of oxygen) = 1 x 12.0u + 2 x 16.0u = 44.0 u. A few further worked examples given in the text follow the identical method: water, H2O, comes to 2 x 1u + 16u = 1 …
Worked out. Problem: find the mass of one molecule of oxygen (O2) in u and in grams. Solution (own words): the molecular mass of O2 is 2 times the average atomic mass of O, i.e. 2 x 16u = 32u, so a single molecule of O2 has a mass of 32.0 u. Converting this to grams using 1 u = 1.66056 x 10^-24 g gives 32.0 x 1.66056 x 10^-24 g = 53.1379 x 10^-24 g. This example shows the two-step process of first finding a molecule's mass on the relative (u) scale straight from its formula, then converting that relative mass into an actual mass …