Which concentration unit is defined as the number of moles of solute per litre of solution?

Correct Option

The correct option is Molarity.

Explanation

Concentration units quantify the amount of solute present in a specific amount of solvent or solution. These units are classified based on whether they use mass or volume to measure the solution's components.

Analysis of Options:

• Molarity Molarity: Correct. Molarity (M) is defined as the number of moles of solute dissolved in one litre of solution. Since it involves volume, it is temperature-dependent.
  Formula: \( M = \frac{\text{Moles of solute}}{\text{Volume of solution in Litres}} \)
• Molality Molality: Incorrect. Molality (m) is defined as the number of moles of solute per kilogram of solvent. Unlike molarity, it is temperature-independent because it is based on mass.
• Normality Normality: Incorrect. Normality (N) is defined as the number of gram equivalents of solute per litre of solution. It accounts for the reactive capacity of a molecule (valency or acidity/basicity).
• Formality Formality: Incorrect. Formality (F) is defined as the number of formula weights of solute per litre of solution. It is technically used for ionic compounds (like NaCl) that do not exist as discrete molecules in solution.

Key Takeaway:
Molarity is a function of volume (moles/Litre), making it sensitive to temperature changes, whereas Molality is a function of mass (moles/kg), making it stable across temperatures.