How many atoms are there in 3.5 moles of arsenic?

In one mole of Arsenic contains 6.02 x 10^23 atoms. This is the Avogadro's number. So in 3.5 moles of Arsenic, there would accordingly be 2.11 x 10^24 atoms.

A mole of As has the Avogadro number of moles. It is equivalent to 6.022 x 10^23. Therefore, 3.5 moles of the element contains 2.11 x 10^24 atoms of Arsenic.