In aqueous solutions, what is true about mg/L and ppm?

• A. mg/L equals ppm.
• B. mg/L is always higher than ppm.
• C. ppm cannot be used for water samples.
• D. ppm is unrelated to mg/L.

Answer: (A)

In dilute aqueous solutions, mg/L and ppm give the same numeric value. This works because ppm is a ratio by mass (parts per million), and at typical conditions 1 liter of water has a mass of about 1 kilogram. So 1 mg of solute per liter of water corresponds to 1 mg per 1,000 g of solution, which is 1 part per million by mass. Therefore mg/L ≈ ppm for water under those conditions.

Slight differences can occur if the density of the solution differs from that of pure water (for example, at high concentrations or different temperatures), but the two units are essentially interchangeable in standard aqueous contexts.