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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!

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Your results are actually very good. The reason for thoroughly washing your copper is to remove any residual water-soluble un-reacted reagents. If the copper were simply dried by sitting in room-air, the bulk of your extra 6% could still be water. That's only 70 mg or 70 ul of water. They way you produced your copper would likely result in a high-surface area product which can be tricky to dry. Ideally, you would do multiple rinses in hot DI water then dry overnight at 105 C (really 70+C should do it). If your yield goes down just a couple percent or so, you can still verify the excess water theory by repeating the drying. If your yield goes down again then you are just having trouble getting all the water out. Make sure you cool the copper, in a desiccator, before weighing.

If you don't have lab time for this here's what I'd try: take you copper home and put it an oven on low, about 200F or so, for several hours. While still hot, place in the smallest reasonably air-tight glass jar you can find an get it back to the balance ASAP! Good luck! Oh, and NEVER touch the copper with your sweaty, oily fingers ;-)

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