15 parts per million (ppm) is equivalent to which concentration in milligrams per liter?

• A. 150 milligrams per liter
• B. 15 milligrams per liter
• C. 150 milligrams per gallon
• D. 15 liters per gallon

Answer: (B)

For dilute aqueous solutions, parts per million and milligrams per liter express the same idea: how much solute is present in a given amount of solution. In water, 1 liter weighs about 1,000 grams (1,000,000 milligrams), so 1 part per million means 1 milligram of solute per liter of solution. With that relation, 15 parts per million corresponds to 15 milligrams per liter.

The other options don’t fit: 150 mg/L would be 150 ppm, which is ten times larger; 150 mg per gallon mixes mass with a different volume unit and isn’t a direct mg/L concentration; 15 liters per gallon is a volume ratio, not a mass concentration.

So 15 ppm equals 15 mg/L.