I have this lab question for the lab called Copper Collection Stoichiometry, where we choose an amount of the limiting reagent (iron) for a reaction between copper (II) sulfate and iron. We are to dissolve copper (II) sulfate in water, making it blue, then add iron, and after filtrating, we should have copper left.

How do we know if iron was the limiting reagent in terms of the visual observations? We made it so that iron was the limiting reagent, but how do we know visually if it is?

I understand that in order for iron to be a limiting reagent, it should've reacted with the solution and all completely turned into copper after the experiment. However how do we know if all the iron actually turned into copper? There is no way that we can visually count every solid flake to make sure there's no iron. I also understand that if iron was the limiting reagent, then there should be still copper (II) sulfate left in the solution, as the reaction didn't completely occur. Is that the answer?

  • $\begingroup$ Throw in more iron and make sure that the reaction starts again. That would verify your last point. $\endgroup$ – Ivan Neretin Nov 15 '17 at 14:47
  • $\begingroup$ @IvanNeretin I don't have the resources to do that, it is a school lab, and we are meant to confirm visually that iron is actually a limiting reagent $\endgroup$ – ImaginingBears Nov 15 '17 at 14:51
  • $\begingroup$ OK, then was the solution at least slightly blue in the end? $\endgroup$ – Ivan Neretin Nov 15 '17 at 14:57
  • $\begingroup$ @IvanNeretin yes slightly blue, although lighter in colour than the original solution with warm water and copper (II) sulfate. Does that indicate there is still copper (II) sulfate in the filtrate? I searched up what Iron (II) sulfate looked like, however it was also slightly blue, mixed with green... $\endgroup$ – ImaginingBears Nov 15 '17 at 15:03
  • $\begingroup$ You got it right. Yes, that counts as a proof. Iron sulfate in comparable concentrations is all but colorless. $\endgroup$ – Ivan Neretin Nov 15 '17 at 15:04

The colour of the filtrate after filtering out the solid is still lightly blue in colour. This indicates that Copper (II) sulfate still is present in the solution, meaning that there was not enough iron to completely react with all the copper (II) sulfate, visually confirming it to be the limiting reagent.

A solution containing only Iron (II) sulfate is colourless, so if the solution is blue in colour, then it shows that Copper (II) sulfate still is present.

Credit to Ivan Neretin for helping me.

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