Which expression correctly represents the general formula for calculating the amount of chlorine bleach needed to achieve a desired ppm, given the percent chlorine in the product?

• A. (ppm/1,000,000) * volume * (100 / % chlorine)
• B. (ppm * volume) / 1,000,000
• C. (ppm/1,000) * volume * (100 / % chlorine)
• D. (ppm/1,000,000) * volume

Answer: (A)

The idea is to convert the desired chlorine amount into how much bleach solution is needed, using the product’s available chlorine percentage and appropriate unit conversions. ppm is mg of chlorine per liter, so the total chlorine you must deliver equals ppm × volume in liters. Each milliliter of bleach provides a certain amount of chlorine: about (percent chlorine as a decimal) × 1000 mg per mL (assuming density ~1 g/mL). So the required bleach volume in milliliters is the total mg of chlorine needed divided by the mg chlorine provided per mL.

If you work through the algebra and express volume in milliliters, you get V_bleach(mL) = (ppm × volume_in_L) / [(percent/100) × 1000], which rearranges to the form (ppm/1,000,000) × volume_in_mL × (100 / percent). This is exactly the expression that yields the needed milliliters of bleach to add.

Example: want 20 ppm in 2000 mL (2 L) with bleach at 6% chlorine. M_needed = 20 mg/L × 2 L = 40 mg chlorine. mg per mL of bleach ≈ 0.06 × 1000 = 60 mg/mL. Volume needed ≈ 40 / 60 ≈ 0.67 mL. Using the formula gives the same result.