How many atoms are found in 12.0 grams of Helium?

One mole of helium has a mass of 4.0 grams. In 12 grams there are 3 moles of He. Each mole contains 6.022x10^23 atoms of helium. Therefore, 3 moles of He will contain 3 times as many atoms or 1.806x10^24 atoms.

Each mole contains Avagodro's number of atoms i. e. 6.023x10^23, so

3 moles x 6.023x10^23 atoms/mole = 18.069x10^23 atoms = 1.8x19^24 atoms