Define molarity and molality and explain when each is used in solution calculations.

• A. Molarity is moles per mole of solution; Molality is moles per liter of solvent; use M when dealing with volume changes.
• B. Molarity is moles per liter of solution; Molality is moles per kilogram of solvent; use M when dealing with volume changes, and m when temperature effects on volume are important or for colligative properties.
• C. Molarity is moles per kilogram of solvent; Molality is moles per liter of solution; use M when dealing with volume changes.
• D. Molarity is moles per liter of solvent; Molality is moles per mole of solvent; use M for dilute solutions.

Answer: (C)

The main idea being tested is how two common ways of expressing concentration differ and when each is most useful.

Molarity is defined as the number of moles of solute in one liter of solution. Because the denominator is the actual volume of the whole solution, molarity depends on the solution’s volume, which can change with temperature. That means if you heat or cool a solution, its volume shifts and so can its molarity.

Molality, on the other hand, is defined as the number of moles of solute in one kilogram of solvent. This uses the mass of the solvent, not the volume of the entire solution, so it remains essentially constant with temperature (volume changes don’t affect it). This stability makes molality particularly useful in calculations where temperature changes are involved or when dealing with properties that depend on the number of dissolved particles relative to solvent, such as colligative properties.

So, use molarity when you’re dealing with volumes at a given temperature and you need the concentration in terms of solution volume. Use molality when temperature could change volumes or when you’re calculating colligative properties, since those rely on the amount of solute per kilogram of solvent rather than per liter of solution.