How many carbon atoms are there in a diamond (pure carbon) with a mass of 55 mg?

1 mol of Carbon = 12 grams.

x mol of Carbon = 55 grams

12*x = 1 * 55

x = 55/12

x = 4.583333 mols of carbon

1 mol of anything is 6.02 * 10^23 atoms

4.58333333 mol = x

1/4.5833333 = 6.02 * 10^23/x

x = 4.58333 * 6.02*10^23

x = 2.7591 * 10^23 Carbon atoms