The mass of copper (Cu) is 63.55 grams.
Remember that 6.022×10²³ atoms = 1 mol (Avogadro's number).
To get from grams to atoms, you need to multiply 1.2 grams of copper × 1 mol of copper × 6.022×10²³ atoms of copper and divide it by 63.54 grams of copper and 1 mol of copper.
This will cancel out the grams and moles to give you the answer in atoms of copper!
1.2 grams of copper × 1 mol of copper × 6.022×10²³ atoms of copper
63.54 grams of copper x 1 mol of copper
= 1.137×10²² atoms of copper
= round to 2 sig figs: 1.1×10²² atoms of copper
