Molality (m) is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent.

m = moles of solute / kilograms of solvent

Example: A 1 m solution of NaCl contains 1 mole of NaCl (58.44 g) dissolved in 1 kg of water.

Unlike molarity, molality is not affected by temperature changes because it's based on mass rather than volume.

The number of moles of a substance can be calculated from its mass and molecular weight using the formula:

moles = mass (g) / molecular weight (g/mol)

Steps:

1. Determine the mass of the substance in grams
2. Find the molecular weight of the substance (sum of atomic weights of all atoms in the molecule)
3. Divide the mass by the molecular weight to get moles

Example: For 10 g of NaCl (MW = 58.44 g/mol):

moles = 10 g / 58.44 g/mol = 0.171 mol

Molarity is used when:

• Working with solutions at constant temperature
• Volume measurements are convenient
• Performing titrations or other volume-based experiments

Molality is used when:

• Temperature may vary (as volume changes with temperature but mass doesn't)
• Working with colligative properties (boiling point elevation, freezing point depression)
• Precise concentration measurements are needed regardless of temperature