How many atoms of copper are present in 1.2 kilogram sample?

For this problem, the atomic mass is required. Take the mass in grams and divide it by the atomic mass. Then multiply it by Avogadro's constant, 6.02 × 1023.

1.2 kg = 1200 g

1200 grams Cu / (63.5 grams) × (6.02 × 1023 atoms) = 1.14 × 1025 atoms