Concentration unit definition:\nThe number of gram-moles of solute dissolved in 1 kg of solvent is called…

Introduction / Context:
Different concentration units suit different applications. Molality is particularly useful in colligative property calculations because it is independent of temperature and pressure, being based on solvent mass rather than solution volume.

Given Data / Assumptions:

• Solute amount expressed in gram-moles (kmol or mol in SI).
• Reference mass of solvent is exactly 1 kg.
• Ambient conditions do not affect the definition since mass is invariant under thermal expansion.

Concept / Approach:
Molality (symbol m) is defined as moles of solute per kilogram of solvent. Contrast this with molarity (moles per litre of solution), normality (equivalents per litre), and formality (formula-gram per litre, used for ionic salts). Because solution volume can vary with temperature, molality is the preferred variable in many thermodynamic relations.

Step-by-Step Clarification:

Verification / Alternative check:
In freezing-point depression, ΔT_f = K_f * m, directly leveraging molality. This temperature independence is why tables and correlations often use molality.

Why Other Options Are Wrong:

• Molarity/Normality/Formality: All reference solution volume, not 1 kg of solvent.

Common Pitfalls:
Confusing “per kg of solvent” with “per litre of solution”; using normality without defining the reaction equivalence correctly.

Final Answer:
molality