Chemistry / Unit 3 Chem 0400 worked example, 60 to 90 seconds
Calculating Molarity from Solute Mass and Solution Volume
The idea
Calculating molarity from a solute's mass means converting that mass to moles using molar mass, then dividing by the solution's total volume, expressed in liters, to get moles of solute per liter.
In practice
A 0.750-L batch of saline contains 13.1 g of sodium chloride, molar mass 58.44 g/mol. Dividing 13.1 g by 58.44 g/mol gives about 0.224 mol of sodium chloride in the batch. Dividing 0.224 mol by 0.750 L gives a molarity of about 0.299 M, the concentration of salt in that particular batch. This two-step pattern, mass to moles and then moles to molarity, applies to any solute once its molar mass and the solution's volume are both known.
Where people slip
Dividing by the solute's mass in grams instead of converting to moles first produces a final number with the wrong units entirely, not merely the wrong numeric value.
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