1 mg/L is equal to which of the following?

• A. 0.1 ppm
• B. 1 ppm
• C. 10 ppm
• D. 100 ppm

Answer: (B)

In water, mg/L and ppm are essentially the same way of expressing concentration because one liter of water has a mass of about one kilogram. That’s 1,000 grams, or 1,000,000 milligrams. So 1 milligram of solute in 1 liter of water represents 1 part in 1,000,000 of the solution, i.e., 1 ppm. Therefore, 1 mg/L equals 1 ppm. The other options would correspond to 0.1 mg/L, 10 mg/L, and 100 mg/L, respectively, which is not the given amount.