You need to prepare 200 mL of a 0.25 M solution from a 1.00 M stock. How much stock is required?

• A. 50 mL
• B. 150 mL
• C. 100 mL
• D. 25 mL

Answer: (A)

When diluting, the amount of solute stays the same, so the concentration times volume before dilution equals the concentration times volume after dilution (M1V1 = M2V2). You want 0.25 M in a final volume of 0.200 L, which means you need 0.25 × 0.200 = 0.050 moles of solute. The stock is 1.00 M, so the volume of stock required is 0.050 moles ÷ 1.00 M = 0.050 L = 50 mL. Measure 50 mL of the stock and add solvent until the total volume reaches 200 mL. This gives the desired 0.25 M solution.