A student has 100.mL of 0.20 M HCl. She takes 5.0 mL of this solution and adds it to 10.0 mL of water. She then takes 3.0 mL of this new solution. What is the concentration of this solution?

Answer 1

There are several ways to solve this problem. Below are given just 2 such approaches.

(1) Find the overall dilution:

5.0 ml —> 10.0 ml = 2 fold dilution

Thus, the concentration is now 1/2 the original, i.e. 1/2 x 0.20 M = 0.10 M

(2) Use V1M1 = V2M2

(5.0 ml)(0.20 M) = (10.0 ml)(x M)

x = (5.0)(0.20)/10.0

x = 0.10 M

NOTE: The fact that one starts with 100. ml and takes 3.0 ml at the end has absolutely no bearing on the final concentration. It would have an effect on the total number of moles of HCl present, but not on concentration.