Given a chlorine demand of 3 mg/L, what would be the residual chlorine if 20 lbs of chlorine gas is used to treat 400,000 gallons of water?

To find the residual chlorine concentration after chlorine treatment, you need to consider both the chlorine demand and the amount of chlorine added.

First, calculate the initial chlorine concentration added from the chlorine gas. Since 1 pound of chlorine gas treats a specific volume of water, you can convert pounds to mg/L. In this scenario, 20 pounds of chlorine is used to treat 400,000 gallons of water.

1 pound equals approximately 453,592 milligrams, so 20 pounds of chlorine is:

20 lbs x 453,592 mg/lb = 9,071,840 mg of chlorine added.

Next, it's necessary to determine the volume of water in liters. There are about 3.78541 liters in a gallon, so:

400,000 gallons x 3.78541 L/gallon = 1,514,152 liters.

Now, to find the concentration of chlorine added in mg/L:

Chlorine concentration = total chlorine added (mg) / total water volume (L), which gives us:

9,071,840 mg / 1,514,152 L ≈ 5.99 mg/L.

This is the initial concentration of chlorine introduced into the water.

Since the chlorine demand is 3