Correct Answer - Option 1 : Molality
Concept:
Molality:
- It is defined as the number of moles of solute per kilogram of solvent.
- Unit of Molality is the mole. Kg-1.
\({\rm{Molality}} = \frac{{{\rm{Moles\;of\;Solute}}}}{{{\rm{Kilogram\;of\;solvent}}}}\)
Molarity:
- It is defined as the moles of a solute per liters of a solution.
- It is also known as the molar concentration of a solution.
- The unit of Molarity is the mole. lit-1.
- It is given by:
\(\rm \frac{{Number\;of\;moles\;of\;solute}}{{Mass\;of\;solvent\left( {in\;g} \right)}} \times 1000\)
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Normality:
- It is defined as the number of grams equivalent per liter of solution.
- Also known as equivalent concentration.
\({\rm{Normality}} = \frac{{{\rm{Gram\;equivalents}}}}{{{\rm{volume\;of\;solution\;in\;liter}}}}\)
Explanation:
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Volume is dependent on temperature. When we increase the temperature, a liquid expands and its volume increases.
- When we decrease the temperature, keeping the pressure constant, the volume of a liquid decreases.
- Thus we can say that volume is directly proportional to temperature.
- So, the concentration terms having volume terms in them will also be dependent on temperature.
- We see that Molarity and Normality are the terms that are dependent on volume.
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Molarity and Normality are both inversely proportional to the volume and are temperature dependent.
- On the other hand, we know that mass does not depend on temperature, so Molality which is the number of moles per kg of a solvent is independent of temperature.
Hence, Molality is temperature independent.