Mixing 5 mL of a 2 M stock with 15 mL of diluent yields what final concentration?

• A. 0.5 M
• B. 2.0 M
• C. 1.0 M
• D. 0.25 M

Answer: (A)

Diluting a solution lowers its concentration in proportion to the total volume added. Use C1V1 = C2V2. You start with 5 mL of a 2 M stock, so the amount of solute is 2 M × 5 mL = 10 milli-moles (in mL units, that’s 0.01 L × 2 M = 0.02 moles, but you can keep it in mL and M). After adding 15 mL of diluent, the total volume becomes 20 mL. The final concentration is C2 = (2 M × 5 mL) / 20 mL = 10 / 20 = 0.5 M. So the concentration drops to one-quarter of the stock because the volume quadrupled (5 mL to 20 mL).

If you see other numbers, they would require different final volumes: 2.0 M would mean no dilution at all; 1.0 M would correspond to a total volume of 10 mL; 0.25 M would need a total volume of 40 mL.