How many moles of argon are present if there are 1.23 x 10^20 atoms?

2.04*10⁻⁴ moles of Ar

Explanation:

We begin from this knowledge:

1 mol of substance has 6.02*10²³ particles

Let's solve this by a rule of three:

6.02*10²³ atoms are contained in 1 mol of Ar

1.23*10²⁰ atoms must be contained in (1.23*10²⁰. 1) / 6.02*10²³ =

2.04*10⁻⁴ moles of Ar