Question

Suppose that, for the reaction Cu(NO3)2 (aq) + 2NaOH (aq) = Cu(OH)2 (s) + 2NaNO3 (aq) , you have not added enough sodium hydroxide to precipitate all the copper as copper hydroxide. a) How could you tell that not all the copper had precipitated? b) What effect would this have on your final yield?

Answer 1

We know that Cu(NO3)2 is clear solution with bright blue color and Cu(OH)2 + NaNO3 is bright blue in color. If we filter the solution after adding some NaOH and see that the filterate is colorless we can say that all the Cu(NO3)2 has precipitated. If the filterate has blue color it means there are still unreacted Cu(NO3)2 present after addition of NaOH.

If we add small amoun of NaOH the final yield of the reaction will be low as we have used limited amount of NaOH and not the required amount of NaOH to precipitate Cu(OH)2 and thus the no. of moles of Cu(OH)2 produced will be lesser than the expected value if would've used enough NaOH.