# What does “at%” mean and how it differs from “mol%”?

Sorry if my question is a bit obvious, but for some reason I'm getting confused with the percentages in this question:

Concentration of $$\ce{A}$$ is $$\pu{80 at\%}$$ and concentration of $$\ce{B}$$ is $$\pu{19.8 at\%}$$ for $$\ce{AB2}.$$ What would be the mass for $$\ce{A}$$ and $$\ce{B}$$ individually, if the atomic weight of $$\ce{A}$$ is $$\pu{5 g/mol}$$ and $$\ce{B}$$ is $$\pu{10 g/mol}?$$

I understand that the molecular weight $$M$$ for my compound would be:

$$M(\ce{AB2}) = (5 + 2\cdot 10)~\pu{g/mol} = \pu{25 g/mol}$$

How do I include the atomic percentage to get the mass of $$\ce{A}$$ and $$\ce{B}?$$

I am just not sure how to use $$\pu{at\%}.$$ My understanding is that if I assume $$\pu{1 mol}$$ of $$\ce{AB2},$$ then I can calculate the mass of $$\ce{A}$$ by assuming that $$\pu{80 at\%}$$ is $$\pu{0.80 mol}?$$

I have another problem where they specify $$\pu{mol\%}$$ (mole percentage) instead. So I am getting confused as to how to use $$\pu{at\%}$$ and $$\pu{mol\%}.$$

• Atom percent is used in X-ray photoelectron spectroscopy or energy dispersive spectrometry. Could you provide some context? Is this an experimental data or just an textbook exercise? – M. Farooq Jun 11 '19 at 3:39
• Any kernel with nucleon number 5 decays extremely fast, so we can definitely exclude option of experimental data. – Poutnik Jun 11 '19 at 4:26
• at% = mol% in case matter is atomised. – Poutnik Jun 11 '19 at 4:42