Question

Five standard solutions of HBr are prepared by serial dilution in which, at each stage, 10.00 mL are diluted to 150.00 mL. Given that the concentration of the most dilute solution is 2.77 × 10−6 M, determine the concentration of the original HBr stock solution.

Answer 1

You know how when you dilute something, you would divide by the volume, and when you take an aliquot, you would multiply by the volume? Well, since you're working backwards, you do the opposite operations. 3.17E-6 M (150.0 mL / 10.00 mL) = 4.76E-5 M (150.0 mL / 10.00 mL) = 7.13E-4 M (150.0 mL / 10.00 mL) = 1.07E-2 M (150.0 mL / 10.00 mL) = 0.160 M So the original stock was a 0.160 M HBr solution.