Parts per million (ppm) is defined as:

Parts per million (ppm) is defined as:

• A. mg of solute / L of solution
• B. g of solute / L of solution
• C. mg of solute / mL of solution
• D. ng of solute / L of solution

Answer: (A)

ppm expresses a mass ratio: the amount of solute per amount of solution, multiplied by 10^6. In dilute aqueous solutions, 1 liter of solution has a mass close to 1 kilogram, so 1 mg of solute in 1 liter gives 1 mg / 1000 g = 1e-3 g / 1e3 g = 1e-6, i.e., 1 ppm. That makes mg of solute per liter of solution the standard way to express ppm. The other units don’t align with that same ratio: grams per liter would be 1000 times larger for the same concentration, milligrams per milliliter is per milliliter rather than per liter, and nanograms per liter would be a thousand times smaller, not matching the typical ppm scale.

ppm expresses a mass ratio: the amount of solute per amount of solution, multiplied by 10^6. In dilute aqueous solutions, 1 liter of solution has a mass close to 1 kilogram, so 1 mg of solute in 1 liter gives 1 mg / 1000 g = 1e-3 g / 1e3 g = 1e-6, i.e., 1 ppm. That makes mg of solute per liter of solution the standard way to express ppm.

The other units don’t align with that same ratio: grams per liter would be 1000 times larger for the same concentration, milligrams per milliliter is per milliliter rather than per liter, and nanograms per liter would be a thousand times smaller, not matching the typical ppm scale.