How many atoms of aluminum are there in a sample with a mass of 100.0 grams?

In order to find the number of moles in a sample we are tasked with first finding the number of moles in that sample. This is because the moles in a substance indicates the amount of the substance that contains the same number of particles as 12 g of the Carbon-12 isotope [or equivalent to 6.02 * 10²³ (which is used as a standard in the world of moles).

Now,

If moles = mass : molar mass

then mol of aluminum = 100 g : 27g/mol

= 3.704 mol

Now that you have calculated the moles then you can do a simple ratio and multiplication at arrive at the number of atoms that would be present in the 100 g of aluminum.

if 1 mol of aluminum contains 6.02 * 10²³ atoms

then let 3.704 mol of aluminum contain x

∴ x = 3.704 mol * (6.02 * 10²³)

= 2.23 * 10²⁴

∴ 100 g of Al would contain ~ 2.23 * 10²⁴ atoms