How can you calculate the mass (in grams) of an isotope?

Question

Can you still use the equation m=nM?
Lets say you have 0.00000245mol of potassium-40. Are you allowed to assume that M=40g/mol and then multiply it by the moles in order to calculate mass?

Is it even possible to calculate the mass of an isotope, or am I losing my mind? (Nuclear chemistry is making me go crazy, seriously).

Answer 1

Yu are doing just fine.

The mass of 40K will be very close to 40 a.m.u, but not spot on. To be more accurate, you need to look it up in a table of nuclei. If you are aiming to use

E = mc2

for energy released, you MUST use the exact mass. But 40 is good enough for most other purposes.

Answer 2

the atomic weights you find in the periodic table already have factored in the % of isotopes

these atomic weight are average atomic weights ; averaged over all naturally occuring isotopes

thats why Chlorine , Cl has a mass of 35.45, it's an average mass

the MW of HCL is then 36.45

the MW of HCl35 it 36

you can see isotopes by mass spec. Br, Cl, and C (C13) are very common isotopes in organic molecules