Molality measures concentration in chemistry, indicating the amount of solute within a given amount of solvent. This article guides you through its definition, formula, and a step-by-step calculation from moles of solute and mass of solvent.
Understanding Molality
Molality is defined as moles of solute divided by the mass of the solvent in kilograms. This concentration is independent of temperature changes. Because molality depends on solvent mass rather than total solution volume, it remains constant even if temperature causes volume to expand or contract.
The standard unit for molality is moles of solute per kilogram of solvent, symbolized by a lowercase ‘m’. For instance, 1.0 m means one mole of solute dissolved in every kilogram of solvent. Molality is a useful concentration unit in specific scientific applications.
The Molality Formula and Its Components
The molality formula is straightforward: molality (m) = moles of solute / kilograms of solvent. It requires two primary components. Moles of solute is the quantity dissolved. If the mass of the solute is provided, convert it to moles by dividing the given mass by the solute’s molar mass.
Kilograms of solvent represents the mass doing the dissolving. Use the solvent’s mass alone, not the entire solution’s mass. If the solvent’s mass is initially given in grams, convert it to kilograms by dividing by 1000, as kilograms are required.
Step-by-Step Calculation Guide
Calculating molality involves clear steps. First, identify the solute and solvent. This clarifies which substance’s moles and mass are needed.
Next, if the amount of solute is given in mass units, such as grams, convert this mass into moles. This conversion requires knowing the molar mass of the solute, found from its chemical formula and the atomic masses of its constituent elements.
Simultaneously, ensure the mass of the solvent is expressed in kilograms. If the solvent’s mass is provided in grams, divide that value by 1000 to convert it to kilograms. Once you have the moles of the solute and the mass of the solvent in kilograms, apply the molality formula by dividing the moles of solute by the kilograms of solvent to obtain molality.
Working Through Examples
Examples illustrate molality calculation. Suppose you dissolve 29.22 grams of sodium chloride (NaCl) in 500 grams of water. First, convert the mass of NaCl to moles. The molar mass of NaCl is 58.44 g/mol. This yields 0.50 moles of NaCl.
Next, convert the mass of the solvent (water) from grams to kilograms. 500 grams is 0.500 kilograms. Applying the molality formula, 0.50 moles of NaCl divided by 0.500 kilograms of water results in 1.0 m molality.
Another scenario: 90.0 grams of glucose (C₆H₁₂O₆) are dissolved in 2.0 kilograms of water. The molar mass of glucose is 180.16 g/mol. This gives 0.50 moles of glucose. Since the solvent’s mass is already in kilograms (2.0 kg), no conversion is needed for water. Dividing 0.50 moles of glucose by 2.0 kilograms of water results in 0.25 m molality.
Molality Versus Molarity
Molality and molarity express solution concentration differently. Molarity is defined as the number of moles of solute per liter of the entire solution. It depends on the total solution volume. Molarity changes with temperature as solution volume expands or contracts.
In contrast, molality is defined as the number of moles of solute per kilogram of the solvent only. Since mass does not change with temperature, molality remains constant regardless of temperature variations. Its temperature independence makes molality useful in experiments with temperature changes. It is preferred for colligative properties (e.g., freezing/boiling point changes) because these depend on solute particles relative to solvent mass, unaffected by volume changes.