By Claire Gillespie, Updated Mar 24, 2022
In a solution, the solute is the minority component that dissolves in the solvent. For instance, salt is the solute in seawater, and isopropanol or ethanol are the solutes in rubbing alcohol.
Moles of solute = mass of solute (g) ÷ molar mass of solute (g mol⁻¹).
A mole (mol) is a unit that counts entities—atoms, molecules, ions, or electrons—using Avogadro’s constant, 6.022 × 10²³ per mole. One mole of any substance has the same number of particles as 12 g of carbon‑12.
To find the molar mass, consult a periodic table. For single‑element solutes, the molar mass equals the element’s atomic weight. For compounds, sum the atomic weights of each constituent. Example: Sodium (Na) = 22.9898 g mol⁻¹, Chlorine (Cl) = 35.4530 g mol⁻¹. The molar mass of table salt (NaCl) is therefore 22.9898 + 35.4530 = 58.4538 g mol⁻¹.
Weigh the solute on an analytical balance. Suppose you dissolve 200 g of NaCl. Dividing the mass by its molar mass (≈58 g mol⁻¹) gives:
200 g ÷ 58 g mol⁻¹ ≈ 3.45 mol of solute.
Molarity (M) is defined as moles of solute per liter of solution. After preparing the solution, measure its total volume. For example, 3.45 mol of NaCl in 10 L of water yields:
3.45 mol ÷ 10 L = 0.345 M.
Thus, the solution’s molarity is 0.345 M.
