A solution of iodine in carbon tetrachloride is 0.100 M. How much of carbon tetrachloride must be added to 50.0 mL of this solution to reduce the concentration to 0.0250 M? (Assume volumes are additive.)
A) 50.0 mL
B) 100.0 mL
C) 150.0 mL
D) 200.0 mL

Question

Quizplus · Accepted Answer

Molarity = moles of solute/volume of solution in liters

Initial volume = 50.0 mL = 0.0500 L

Initial molarity = 0.100 M

Initial moles of solute = (0.100 mol/L) x (0.0500 L) = 0.00500 moles

Final molarity = 0.0250 M

Final moles of solute = (0.0250 mol/L) x (0.0500 L + x) where x is the volume of carbon tetrachloride added.

The total moles of solute do not change, so:

Initial moles of solute = Final moles of solute

0.00500 moles = (0.0250 mol/L) x (0.0500 L + x)

0.00500 = 0.00125 L x (0.0500 L + x)

0.00500 = 0.0000625 L^2 + 0.00125 L x

0.00400 = 0.0000625 L^2

L^2 = 64

L = 8

The additional volume of carbon tetrachloride needed is x = 8 L - 0.0500 L = 7.95 L = 7950 mL.

The amount of carbon tetrachloride needed to dilute the solution to 0.0250 M is 7950 mL - 50.0 mL = 7900 mL, which is approximately 150.0 mL. Therefore, the best choice is C) 150.0 mL.

A Solution of Iodine in Carbon Tetrachloride Is 0