To calculate the mass of 0.0420 moles of copper, we need to use the molar mass of copper. The molar mass of copper is 63.55 grams per mole.

Mass = moles × molar mass

Mass = 0.0420 moles × 63.55 grams per mole

Mass = 2.67 grams

Therefore, the mass of 0.0420 moles of copper is 2.67 grams.