By Chris Deziel
Updated Aug 30, 2022
A solution is a homogeneous mixture where a solute is dispersed within a solvent. In most everyday solutions, water serves as the solvent, and the solute determines the solution’s properties. Chemists need a precise way to describe solute concentration, and the most widely used unit is molarity (M), defined as the number of moles of solute per liter of solution.
The mole is a foundational measurement in chemistry, anchored by Avogadro’s number (6.022 × 1023 atoms or molecules). One mole of any substance has a mass, in grams, equal to its molar mass (the atomic or molecular weight in atomic mass units). For example, hydrogen’s atomic mass is 1.008 amu, so one mole of hydrogen weighs 1.008 grams.
To determine the molar mass of a compound, add the atomic masses of all atoms in its formula (found on the periodic table). Once you have the molar mass, you can convert a weighed sample to moles by dividing its mass by the molar mass.
Example: Sodium hydroxide (NaOH)
Na (22.990 g/mol), O (15.999 g/mol), H (1.008 g/mol). Molar mass = 22.990 + 15.999 + 1.008 ≈ 40.0 g/mol. A 32‑gram sample contains 32 g ÷ 40.0 g/mol ≈ 0.80 moles.
With the moles of solute known, you need the solution’s volume in liters. If you measure volume in another unit, convert it using:
Molarity (M) = moles of solute ÷ liters of solution.
Calculate the molarity of a solution prepared by dissolving 12 g of sodium chloride (NaCl) in 20 oz of water.
Na (23.0 g/mol) + Cl (35.5 g/mol) = 58.5 g/mol. Moles = 12 g ÷ 58.5 g/mol ≈ 0.205 moles.
20 oz ÷ 33.81 oz/L ≈ 0.592 L.
0.205 moles ÷ 0.592 L ≈ 0.35 M.
Thus, the resulting solution has a molarity of roughly 0.35 M.
