By Oxana Fox
Updated Mar 24, 2022
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In chemistry, concentration can be expressed in many ways. Molarity tells you how many moles of a solute are present in one liter of solution, while weight percentage indicates the mass of solute relative to the total mass of the solution. Converting between these two units is essential for preparing solutions, interpreting experimental data, and solving textbook problems.
Look up the atomic masses of each element in the dissolved compound on the Periodic Table. For potassium chloride (KCl), potassium (K) has an atomic mass of 39 u and chlorine (Cl) 35.5 u.
Multiply each atomic mass by the number of atoms of that element in the formula, then add the results. For KCl:
39 × 1 + 35.5 × 1 = 74.5 g mol⁻¹.
Multiply the molar mass by the molarity to find how many grams of solute are present in one liter. A 0.5 M KCl solution contains 74.5 g mol⁻¹ × 0.5 mol L⁻¹ = 37.25 g of salt.
Use the solution’s density to calculate its mass per liter. If the density is 1.1 g mL⁻¹, then 1 L (1,000 mL) weighs 1.1 × 1,000 = 1,100 g.
Divide the solute mass by the total solution mass and multiply by 100:
(37.25 g ÷ 1,100 g) × 100 = 3.39 % w/w.
