In my grade 11 chemistry class, we performed a lab in which we added 1.00g of iron to 6.00g of copper (II) sulphate which was dissolved in water. Using stoichiometry, we predicted that if the iron was ferrous iron, 1.14g of copper would be produced and if the iron was ferric iron, 1.71g of copper would be produced. We waited 24 hours for the copper to dry and then we weighed the copper. The copper had a total mass of 1.24g. We then asked our teacher how this could be as we could not have ended up with more copper than 1.14g (as she already told us that the copper was ferrous). She told us that we need to make sure we rinse the copper really well.
So we decided to do a second trial. We were extra careful in our measurements and we made sure to rinse the copper well with distilled water. We waited 24 hours for the copper to dry and weighed the copper. The copper had a total mass of 1.21g. This was an improvement, but we also understood that we cannot have a percentage yield of over 100%, while ours was 106%.
What could have caused the copper to have a mass larger than predicted? Also, what was the reasoning behind my chemistry teacher advising us to rinse the copper before we let it dry?
Note: I can provide photos and calculations, as well as any other information if it is needed. Thank you!