Twenty-seven pounds (27 lb) of chlorine gas is used for treating 750,000 gal of water each day. The chlorine used up by the microorganisms in the water is measured to be 2.6 mg/L. What is the residual (excess) chlorine concentration in the treated water?

The correct answer is 1.778 milligrams per liter.

Explanation:

Based on the given information, the concentration of chlorine gas given is 27 pounds or 27*453.59 = 12327.93 grams.

The quantity of water given is 750000 gallons or 750000 * 3.754 = 2815500 L

The concentration of chlorine used up by the microorganism is 2.6 mg/L or 0.0026 gm/L.

Thus, for 2815500 L of water, the amount of chlorine used is,

= 2815500 * 0.0026

= 7320.3 grams

The left quantity of chlorine is 12327.93 - 7320.3 = 5007.63 grams

The concentration of residual chlorine is,

= 5007.63 g/2815500 = 0.00177859 or 1.778 mg/L.