Here's how to calculate the number of atoms in 3.5 moles of arsenic:

1. Avogadro's Number

* Avogadro's number is a fundamental constant in chemistry: 6.022 x 10²³ atoms/mol. This means there are 6.022 x 10²³ atoms in one mole of any substance.

2. Calculation

* Multiply the number of moles by Avogadro's number:

3.5 moles As * (6.022 x 10²³ atoms/mol) = 2.1077 x 10²⁴ atoms

Answer: There are approximately 2.1077 x 10²⁴ atoms in 3.5 moles of arsenic.