Question

How many atoms are in a sample of 68.7 g copper (Cu)?

Answer 1

Divide the number of grams present in the sample by copper's gram atomic weight to find the number of gram atomic weights present. Then multiply that result by Avogadro's Number: 6.022137 x 10^23 atoms/gram atomic weight.1,200 g/(63.54 g/gram atomic weight) ? 18.885741 gram-atomic weights. Hope this helps. 

Answer 2

First convert grams into moles.
Then convert moles to atoms.

There are 63.546 grams of copper per mole. (This is just copper's atomic weight)
Thus, a 68.7 gram copper sample is (68.7/63.546) mols = 1.081 mols.

There are 6.022 * 10^23 atoms per mol.
6.022 * 10^23 * 1.081 = 6.52 * 10^23