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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