How to Calculate a Solution's Mass


The mass of solute dissolved in a solution depends on the solution's concentration. A number of ways exist for describing this concentration, including the solute's percentage by mass and the number of particles dissolved in a mass of solvent. But the most common measure of concentration is a solution's molarity, which describes the moles of particles dissolved in one liter of solution. The mass of a single mole of a substance equals its relative formula mass, so the total mass in the solution is the product of this relative mass and the number of moles in it.

  • Calculate the substance's relative formula mass by multiplying each element's atomic weight by the number of its atoms in the formula. For a list of atomic weights, see the Resources section. Each molecule of potassium chloride, for instance, contains an atom of potassium, which has a relative mass of 39, and an atom of chlorine, which has a relative mass of 35.5: (39 --- 1) + (35.5 --- 1) = 74.5.

  • Multiply the solution's rated molarity by its volume. If, for example, it has a molarity of 2.0 and measures 0.35 liters in volume: 2.0 --- 0.35 = 0.7. This is the solute's quantity, measured in moles.

  • Multiply together the number of moles of solute and its relative mass: 0.7 --- 74.5 = 52.15. This is the mass of solute in the solution, measured in grams.

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