solving limiting reactant problems in solution

Question

Suppose 3.50g of zinc bromide is dissolved in 300.mL of a 65.0mM aqueous solution of potassium carbonate.Calculate the final molarity of bromide anion in the solution. You can assume the volume of the solution doesn't change when the zinc bromide is dissolved in it.

Answer 1

First, write down the correctly balanced equation for the reaction taking place:

ZnBr₂(aq) + K₂CO₃(aq) ==> ZnCO₃(s) + 2KBr(aq)

Next, find the limiting reactant so that we can predict the amount of KBr that will form.

moles ZnBr₂ used = 3.50 g x 1 mole/225 g = 0.0156 moles ZnBr₂

moles K₂CO₃ = 0.3 L x 0.065 mole/L = 0.0195 moles K₂CO₃

Since in the balanced equation they react in a 1:1 mole ratio, the ZnBr₂ is limiting

Predict moles of KBr formed under these conditions.

0.0156 moles ZnBr₂ x 2 moles KBr/1 mole ZnBr₂ = 0.0312 moles KBr formed = 0.0312 moles Br^-

Find the final molarity of the bromide anion (Br^-).

Final volume = 300.0 ml = 0.300 L

moles Br^- = 0.0312

Final molarity Br^- = 0.0312 moles/0.300 = 0.104 M (to 3 significant figures)