Molarity and Dilution: Tracking Solute Through a Solution
Understand what molarity measures, why dilution conserves solute, and how unit discipline prevents common errors.
Molarity is amount per final volume
Molarity connects the microscopic amount of solute with the macroscopic volume of the finished solution. The denominator is the total solution volume—not the amount of solvent poured in before mixing.
Because the SI-compatible definition uses moles per cubic decimeter, classroom calculations normally express volume in liters.
Dissolving 0.250 mol and making the final volume up to 0.500 L gives M = 0.500 mol/L.
From mass to concentration
Laboratory problems often provide solute mass rather than amount in moles. Divide mass by molar mass first, then divide the resulting amount by final solution volume.
Keep molar mass in grams per mole when the measured mass is in grams; the gram units cancel before molarity is calculated.
Why the dilution equation works
Adding solvent changes volume and concentration but does not change the amount of solute transferred. Before and after dilution, n = CV. Setting the two solute amounts equal gives the dilution equation.
Both concentrations must use compatible units, and both volumes must use the same unit. Liters are not mandatory here because the matching volume units cancel.
To dilute 25.0 mL of 2.00 M stock to 0.500 M: V₂ = 2.00 × 25.0 / 0.500 = 100 mL.
When the shortcut does not apply
The equation describes dilution without reaction or solute loss. It does not replace reaction stoichiometry, account for evaporation, or guarantee that volumes are exactly additive in non-ideal mixtures.
For laboratory preparation, uncertainty, volumetric-glassware tolerances, temperature, and safety procedures remain important.
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Molarity and dilution calculatorReferences
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Educational article, not professional advice. Editorial policy · Calculation methodology