Which expression is used for calculating dilutions?

• A. PV = nRT
• B. (C1)(V1) = (C2)(V2)
• C. E = mc^2
• D. F = ma

Answer: (B)

When diluting a solution, the amount of solute present doesn’t change. That means the product of concentration and volume before dilution equals the product after dilution. This is why the expression used is C1V1 = C2V2. It lets you relate the initial concentration and volume to the final concentration and volume, assuming the final volume is reached by adding solvent.

For example, if you take 2.0 mL of a 10 M stock and dilute it to a total volume of 50.0 mL, the final concentration is C2 = (C1V1)/V2 = (10 M × 2.0 mL) / 50.0 mL = 0.4 M.

The other formulas come from different physics and chemistry concepts and aren’t used for calculating dilutions: PV = nRT describes the behavior of gases; E = mc^2 is mass–energy equivalence; F = ma is Newton’s second law.