Bench Primer

Bench Primer is a short series on measurement in the chemistry lab: the nine things that trip people up between a data sheet and a weighing boat. Each primer starts from a definition, shows the arithmetic, and links the record every constant comes from. The aim is the page you wish had been at the front of the lab notebook.

The primers are reference material. They describe how quantities are defined and computed; they are not advice about anything, and they do not discuss the use of any substance.

Every mass on the site is computed when the page is rendered, from one set of constants: the IUPAC abridged standard atomic weights for average masses, and the exact masses of the most abundant isotopes for monoisotopic masses. The tables, the primers and the calculators all read the same constants, so they cannot disagree with one another. Where another source differs in the third decimal of an average mass, the cause is the atomic-weight table it chose, and the primers say so.