Sources of Error:

Are you under, or over?

Under. Congratulations you are human. Bits of your chemicals have slipped away during the lab. Now where did they go.

Not transferred from the weighing boat. Probably only accounts for a few 0.01 to 0.001 g.

Not rinsed from the reaction beaker. Error on the 0.1 to 0.01 gram scale. Weigh your dry dirty beaker. Then wash your beaker, dry it well and weigh it. Much better if you had thought to do this before the reaction. Because washing could clean your beaker cleaner than it was in the beginning, or may leave behind some paper towel dust making it heavier. Some data may be better than no data.

Incomplete reaction: your filtered water would remain light blue from the unreacted Cu(OH)₂ This could be a substantial error. Easily in the 0.1 range or better. I did not observe this in any lab group

Incomplete filtration: brown gunk in the erlenmeyer flask as a result of passing through the relatively coarse filter paper. I did not observe this in any lab group. This could be a big error.

Over. If you have more filtrate than is theoretically possible you have big problems. Several possibilities pop to mind. You or a lab partner may be an Arabian Jin, or rejects from Hogwarts or some other mystical being not bound by normal laws of Chemistry or boring stuff like that. More likely you made and oopsie somewhere.

Incorrect math in the factor label problems. Check molar masses (GFW's) and calculations. Who knows how big this could be. It could be positive or negative error.

Wet filtrate. Your stoichiometric calculations assumed the filtrate would be perfectly dry. So if your filter cake is moist, it is too heavy.

Impure filtrate/incomplete rinsing. This is probably the number 1 problem. If you see white crusty or fluffy crystals at the edge of the filter paper, you have precipitated NaOH and Na₂SO₄ that we wanted to discard. As the water evaporates, it is forced to leave behind the soluble NaOH and Na₂SO₄. Just rinse the filtrate again a couple of times and re-dry the filtrate.

If your sure that you haven't made any of these mistakes, and you still have made more CuO than is possible you are truly a magician and we should start trying to do something really important like filtering Au from seawater. Yahooo!!!!$$$$$

Limiting reagent: this is the reactant that controls the reaction. When this material runs out, the reaction must stop. You are then left with an excess of other reactants.

Would our limiting reagent be the CuSO₄ x 5H₂O or the NaOH ?

Which to we have excess of (this one is not the limiting reagent.

When in doubt about limiting reagent. Do both 4-step factor-label problems with the amounts present and whichever produces less product is the limiting reagent.

Solubility Rules: apply the solubility rules to each reaction in this lab.

CuO Ksp = 10^-19.7

Cu(OH)2 = 2.6 x 10 ^-19

NaOH and Na₂SO₄ are expected to be fully soluble because...

However what happens to the dissolved Na⁺ and SO₄ ^2- ions when the water evaporates? What does this say about the solubility of most solids in gases (water vapor in this case)?