Which statement about mg/L and ppm is true?

• A. They represent the same concentration in dilute aqueous solutions
• B. They are unrelated units
• C. 1 mg/L equals 1000 ppm
• D. ppm is always smaller than mg/L

Answer: (A)

mg/L is a mass per volume measure, while ppm is a mass per mass ratio. In dilute aqueous solutions, the density of water is about 1 g/mL, so 1 liter of solution weighs roughly 1 kilogram. That makes 1 mg of solute per liter (1 mg/L) essentially the same as 1 mg per kilogram of solution (1 mg/kg), which is 1 part per million. So, in these conditions, mg/L and ppm express the same concentration value. If the solution’s density differs significantly from water, the equivalence isn’t exact, and you’d adjust using the actual density. The other statements aren’t correct because they either imply no relation, give an incorrect conversion like 1 mg/L equals 1000 ppm, or claim ppm is always smaller than mg/L, which isn’t true in dilute aqueous solutions.