By Michael Judge
Updated Aug 30, 2022
Scientists rely on molarity (M) to express the concentration of a chemical solution. Molarity is defined as the number of moles of a solute per liter of solution. A mole represents a large quantity—6.02 × 10²³ molecules—so molarity offers a convenient way to quantify solution strength. To determine molarity, you need the mass of the solute and the total volume of the finished solution.
Enter the mass of the chemical that was dissolved in the solvent. The mass must be expressed in grams. If you have the mass in another unit (e.g., ounces or pounds), convert it to grams before proceeding.
Divide the solute mass by its molecular weight, which is expressed in grams per mole. The quotient gives the number of moles of the compound present in the solution.
Divide the moles calculated in Step 2 by the total volume of the solution, expressed in liters. The result is the molarity (M), in moles of solute per liter of solution.
Students often mistakenly divide by the volume of the solvent alone. Always divide by the final mixture’s volume, which typically exceeds the solvent volume. The “molecular weight” used here is often called the gram‑molecular weight.
