When diluting a high‑concentration alcohol to a lower one, the classic formula C1 × V1 = C2 × V2 is used, where C1 and V1 are the starting concentration and volume, and C2 and V2 are the desired concentration and final volume. Here we start with 100 % isopropyl alcohol and aim for 500 mL of a 70 % solution.
Before handling any chemicals, don safety goggles and gloves. Closed‑toe footwear is also recommended.
Set C1 = 100 % for the stock alcohol and calculate V1 using the equation:
V1 = (C2 × V2) ÷ C1
With C2 = 70 % and V2 = 500 mL, this becomes:
V1 = (70 × 500) ÷ 100 = 350 mL
Pour 350 mL of pure isopropyl alcohol into the 500 mL graduated cylinder. Read the meniscus at eye level to ensure accuracy.
Top up the cylinder with 150 mL of distilled water, again measuring to the eye level of the meniscus. The total volume should now be exactly 500 mL.
Carefully pour the 70 % solution into a clean beaker marked “70 % Isopropyl Alcohol.” Stir with a glass rod until homogeneous.
Use C1 × V1 = C2 × V2 to calculate the volume of 100 % alcohol needed; for a 500 mL 70 % solution, mix 350 mL alcohol with 150 mL water.
Isopropyl alcohol is flammable and can irritate skin and eyes. Always work in a well‑ventilated area, wear appropriate PPE, and keep the solution away from open flames.
