Converting between parts per million (ppm) and molarity is a common task in various fields, including water quality analysis, aquarium maintenance, horticulture, and chemical production. Understanding these concentration units is crucial for accurate measurements and solutions. Let’s explore the concepts of ppm and molarity in more detail.

Understanding ppm and Molarity:

Parts Per Million (ppm):

  • Ppm is a measure of concentration, indicating the number of parts of a substance in a million parts of another.
  • In the context of water solutions, 1 ppm is often approximated as 1 mg of substance per liter of water (1 mg/L).
  • For diluted solutions at room temperature, where water density is considered 1 g/mL, ppm can be expressed as 1 mg in 1,000,000 mg (1 part per million).


  • Molarity is the molar concentration, representing the number of moles of a substance in one liter of solution.
  • Avogadro’s number (6.0221409 × 10^23) indicates the number of molecules or particles in one mole.

Technical Definition of ppm:

  • Ppm, despite its name, is often represented as mg/L when dealing with solutions.
  • The ppm calculation involves expressing the mass of a solute in grams for every million grams of the solution.

Conversion from ppm to Molarity:

  • To convert ppm to molarity, use the formula: ppm = (moles/L) × (molar mass) × 1000.
  • Example: Seawater, with a molarity of 0.599 M NaCl, is approximately 1750 times saltier than drinking water with a guideline of 20 ppm NaCl.

Preparing a Solution Example:

  • If you have a 1 M NaOH stock solution and want a 200 ppm NaOH solution, follow these steps:
    1. Convert 200 ppm to molarity.
    2. Use the dilution equation to determine the volume of stock solution needed for the desired molarity.


  • Calculate molarity with density and ppm: Divide solution density by the solute’s molar mass. For ppm, substitute density with ppm and multiply by 1000 mg/g.
  • Grams and ppm: One gram in a one-liter solution equals 1000 ppm.
  • Calculate ppm from volume concentration: Multiply molar concentration by molar mass, divide by solute density, and multiply by 1000 mg/g.

Example Problem Solved:

  • Concentration of 0.5 moles of CH₄ in 1500 ml of water is 5,333 ppm, calculated by molar concentration, molar mass, and conversion to ppm.

This information provides a comprehensive guide for understanding and applying ppm and molarity conversions in various scenarios.