Question

Calculate the expected isotope distribution for alanine at natural abundance. (Hint: Only consider carbon as it will have the largest affect on isotope distributions. Itsbest to calculate them as a percentage.)

Answer 1

The average atomic mass (also called the average atomic weight or just atomic weight ) of an element is defined as the weighted average of the masses of all its naturally occurring stable isotopes.
For example, for carbon: ¹²C = 12 Dalton (exact) or 98.90% and
¹³C = 13.003355 Dalton or 1.10%

The average atomic mass of carbon is calculated as:

(98.9% X 12.0 + 1.1% X 13.003355) /100%
              = 12.011
For most purposes such as weighing out bulk chemicals only the average molecular mass is relevant since what one is weighing is a statistical distribution of varying isotopic compositions.