Molality and Osmolality in Solution Chemistry: Definitions, Calculations, and Applications

Molality and Osmolality in Solution Chemistry

Molality (m)

Molality is a concentration unit used in chemistry to express the amount of solute dissolved in a given mass of solvent. Unlike molarity, molality is independent of temperature because it is based on mass rather than volume.

• Definition: Molality (m) is the number of moles of solute per kilogram of solvent.

• Formula:

$m = \frac{\text{moles of solute}}{\text{kg of solvent}}$

• Key Properties:

  • Temperature independent (mass does not change with temperature).

  • Useful for colligative property calculations (e.g., boiling point elevation, freezing point depression).

Example Calculation

Example: A solution contains 24.8 g of sucrose (C12H22O11) dissolved in 550.0 g of water. Calculate the molality of the solution.

• Step 1: Calculate moles of sucrose:

  • Molar mass of sucrose = 342 g/mol

  • Moles of sucrose = $\frac{24.8\ \text{g}}{342\ \text{g/mol}} = 0.0725\ \text{mol}$

• Step 2: Convert solvent mass to kg:

  • 550.0 g water = 0.550 kg

• Step 3: Calculate molality:

  • $m = \frac{0.0725\ \text{mol}}{0.550\ \text{kg}} = 0.132\ \text{mol/kg}$

Practice Questions

• Question 1: A solution is prepared by dissolving 43.0 g potassium chlorate (KClO3) in enough water to make 100.0 mL of solution. If the density of the solution is 1.760 g/mL, what is the molality of KClO3 in the solution?

  • Answer: 3.55 m

• Question 2: The density of a 15.7 M methanol (CH3OH) solution is 0.858 g/mL. If H2O is the solvent, what is the molality of the solution?

  • Answer: 18.3 m

Osmolality

Osmolality is a measure of the total number of dissolved particles (ions or molecules) in a solution, expressed as osmoles per kilogram of solvent. It is especially important in biological and medical contexts.

• Definition: Osmolality is the number of osmoles of solute per kilogram of solvent.

• Formula:

$\text{Osmolality} = (\text{number of particles per formula unit}) \times m$

• Key Properties:

  • Accounts for dissociation of ionic compounds (e.g., NaCl dissociates into Na+ and Cl-).

  • For covalent compounds, the number of particles is usually 1 (no dissociation).

Example Calculation

Example: What is the osmolality of total ions in an aqueous solution prepared by dissolving 0.400 moles of Pb(NO3)2 in 750.0 g water?

• Step 1: Calculate molality:

  • 750.0 g water = 0.750 kg

  • $m = \frac{0.400\ \text{mol}}{0.750\ \text{kg}} = 0.533\ \text{mol/kg}$

• Step 2: Count total ions:

  • Pb(NO3)2 dissociates into 1 Pb2+ and 2 NO3- ions (total = 3 ions per formula unit)

• Step 3: Calculate osmolality:

  • $\text{Osmolality} = 3 \times 0.533 = 1.60\ \text{osmol/kg}$

Practice Questions

• Question 1: What is the ionic molality of sodium ions in a solution of 25.7 g NaNO3 dissolved in enough water to make a 150.0 mL solution? Density of the solution is 1.02 g/mL.

  • Answer: 2.57 m

Comparison Table: Molality vs. Osmolality

Property	Molality (m)	Osmolality
Definition	Moles of solute per kg solvent	Osmoles of particles per kg solvent
Considers Dissociation?	No	Yes (counts total particles)
Units	mol/kg	osmol/kg
Temperature Dependence	Independent	Independent
Application	Chemical solutions, colligative properties	Biological fluids, medical analysis

Additional info: Osmolality is especially important in physiology and medicine, where the concentration of dissolved particles affects osmotic pressure and cell function.