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If 6.022 × 10²³ atoms = 1 mole
Then, moles of Cu = 1.5 x 1020/6.022 × 1023 = 0.00025 moles
Mass of Cu in coin = = 0.00025 mol × 64g/mol = 0.016 g
Therefore 1.5 x 1020 atoms of copper has a mass of 0.016 g.
This 80% of the piece of metal, weight of the mass is 100% = 100/80 x 0.016 = 0.02 g
The piece of metal has a mass of 0.02g