Nominal Mass versus Exact Mass

1. Nominal Mass Determination of Carbon Monoxide

Determining the nominal mass is easy. Just use the average isotopic mass of each element (note: periodic tables use average isotopic mass). Thus,

nominal mass = 12.01 + 16.00 = 28 (rounding off to units place)

2. Exact Mass Determination of Carbon Monoxide

Carbon Monoxide in nature consist of several isotopic species. The exact mass is the most abundant isotope. Thus, for carbon monoxide the most abundant isotope consist of carbon-12 and oxygen-16. Use page 28 of the course reader or the below table for determining the exact mass.

exact mass = 12.0000 + 15.9949 = 27.9949

Isotope	Atomic Weight
Hyrdogen-1	1.007825
Hydrogen-2 (deuterium)	2.014102
Carbon-12	12.00000
Carbon-13	13.003354
Nitrogen-14	14.003074
Oxygen-16	15.994915