Ratio of reactants

Question

The following balanced equations represent double replacement reactions that each result in the formation of a precipitate. For each reaction, which ratio of reactants would result in the formation of the greatest amount of precipitate?

Cu(NO3)2 (aq) + 2NaOH(aq) --> Cu(OH)2 (s) + 2NaNO3 (aq) Note: Cu(OH)2 is a blue precipitate

FeSO4 (aq) + 2NaOH(aq) ---> Fe(OH)2 (s) - Na2SO4 (aq) Note: Fe(OH)2 is a dark green precipitate

Fe(NO3)3 (aq) + 3NaOH (aq) ---> Fe(OH)3 (s) + 3NaNO3 (aq) Note: Fe(OH)3 is a red-orange precipitate

Answer 1

Not entirely sure what the question is getting at. If the topic is "limiting reactants", then you want the reactant that contains the element of the precipitate to not be limiting. For example ...

Cu(NO₃)₂ (aq) + 2NaOH(aq) --> Cu(OH)₂ (s) + 2NaNO₃ (aq)

The precipitate is Cu(OH)₂ so in order to obtain the maximum amount of this compound, you want NaOH to be in excess so that ALL of the Cu is precipitated as Cu(OH)₂. This means the ratio of reactants should be at least 2:1 NaOH to Cu(NO₃)₂.

If this is what the question is asking, then the others would follow suit. Thus, you'd have...

FeSO₄ (aq) + 2NaOH(aq) ---> Fe(OH)₂ (s) + Na₂SO4 (aq)

Precipitate contains Fe so you want NaOH to be in excess so ALL the Fe is precipitated. The ratio of reactants should be at least 2:1 NaOH to FeSO₄

Fe(NO₃)₃ (aq) + 3NaOH (aq) ---> Fe(OH)₃ (s) + 3NaNO₃ (aq)

Precipitate contains Fe so you want NaOH to be in excess so ALL the Fe is precipitated. The ratio of reactants should be at least 3:1 NaOH to Fe(NO₃)₃