If a water sample measures 2 mg/L, what is the equivalent concentration in ppm?

• A. 2 ppm
• B. 0.2 ppm
• C. 20 ppm
• D. 200 ppm

Answer: (A)

In water, mg/L is essentially the same as ppm for dilute solutions because 1 liter of water has a mass of about 1 kilogram. That makes 1 mg per liter correspond to 1 part per million. So, 2 mg/L equals 2 ppm.

The other numbers would require different amounts of solute (0.2 mg/L would be 0.2 ppm, 20 mg/L would be 20 ppm, and 200 mg/L would be 200 ppm), which are not the given concentration.