2) You start with 3.8 x 1021 gold atoms, how many moles is this?

0.0063mol

Explanation:

Number of moles (n) in a substance can be calculated by dividing the number of atoms (nA) of that substance by Avagadro's number i.e. 6.02 × 10^23. That is; n = nA ÷ 6.02 × 10^23

1 mol of Gold = 6.02 × 10^23 atoms

Therefore, 3.8 x 10^21 atoms of Gold will be;

3.8 x 10^21 ÷ 6.02 × 10^23

= 3.8 ÷ 6.02 × 10^ (21 - 23)

= 0.6312 × 10^-2

= 0.0063mol of Gold

the answer is option a.