Calculating Molarity and Molality in Chemistry Solutions
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Molarity and molality are two important ways to express solution concentration. Molarity measures moles of solute per liter of total solution, while molality measures moles of solute per kilogram of solvent only. Understanding the difference between these two concentration units is crucial for accurate chemical calculations.
The molarity formula is moles of solute divided by liters of solution. Let's calculate an example: 0.25 grams of sodium hydroxide dissolved in water with a final volume of 25 milliliters. First, we convert grams to moles by dividing by the molecular weight of 40 grams per mole, giving us 0.00625 moles. Then we divide by the volume in liters, 0.025 liters, to get 0.25 molar.
The molality formula is moles of solute divided by kilograms of solvent. Using the same example, we have 0.00625 moles of sodium hydroxide added directly to 25 milliliters of water. Since water has a density of 1 gram per milliliter, 25 milliliters equals 25 grams or 0.025 kilograms. Dividing 0.00625 moles by 0.025 kilograms gives us 0.25 molal.
There are several key differences between molarity and molality. Molarity uses liters of total solution in the denominator, while molality uses kilograms of solvent only. Molarity is temperature dependent because solution volume changes with temperature, but molality is temperature independent since mass doesn't change. Molarity is more common in laboratory work, while molality is preferred for studying colligative properties like freezing point depression.