If you start with 100 mL of stock formaldehyde and want to make 10% formalin, how much water should be added?

• A. 0 mL
• B. 900 mL
• C. 100 mL
• D. 1100 mL

Answer: (B)

Dilution relies on keeping the amount of solute the same while increasing the total volume. You have 100 mL of formaldehyde in the stock, and you want the final solution to be 10% formaldehyde. So that 100 mL must represent 10% of the final volume, meaning final volume = 100 mL / 0.10 = 1,000 mL. To reach 1,000 mL total, add 1,000 − 100 = 900 mL of water. This yields a 10% formalin solution. Adding 0 mL, 100 mL, or 1,100 mL would not give 10% as required.