A small tank containing 8,500 gal of water needs to be disinfected to be put back in service. If the dosage needed is 50 mg/L, how many pounds of sodium hypochlorite that has 12.5% available chlorine are required?

• A. 12
• B. 24
• C. 28
• D. 2,800

Answer: (C)

You determine the amount by converting the disinfection dose from mg per liter to total milligrams for the whole tank, then convert that to a mass of sodium hypochlorite using the available chlorine fraction.

First, convert the tank volume to liters: 8,500 gallons × 3.78541 L/gal ≈ 32,176 L. The total chlorine required is 50 mg/L × 32,176 L ≈ 1,608,800 mg = 1,608.8 g ≈ 1.609 kg.

Sodium hypochlorite here has 12.5% available chlorine, so the mass of hypochlorite needed is 1.609 kg ÷ 0.125 ≈ 12.87 kg. Convert to pounds: 12.87 kg × 2.20462 ≈ 28.4 lb, which rounds to about 28 pounds.

So, about 28 pounds of sodium hypochlorite are needed.