This item can be answered by using the concept of material balance. The amount of substance used for the reaction should be the same before and after the reaction occurred.
Amount of reactants = 3.55 grams + 4.60 grams = 8.15 grams
We are also given that the amount of product accounted for is only 6.25 grams, this entails that not all copper was reacted. Subtracting this value from 8.15 g will give us an answer of 1.9 grams.
Answer: 1.9 g of copper
