Question

Explain the difference between molarity and molality. Under what circumstances would molality be a more accurate measure of the concentration of a prepared solution than molarity? Why?

Short answer

Molality is preferred over molarity when temperature or pressure changes because molality depends on the mass of the solvent, which remains constant.

Step-by-step solution

Title - Define Molarity

Molarity (M) is a measure of the concentration of a solute in a solution. It is defined as the number of moles of solute per liter of solution. The formula for molarity is: \[ M = \frac{\text{moles of solute}}{\text{liters of solution}} \]

Title - Define Molality

Molality (m) is another measure of the concentration of a solute in a solution. It is defined as the number of moles of solute per kilogram of solvent. The formula for molality is: \[ m = \frac{\text{moles of solute}}{\text{kilograms of solvent}} \]

Title - Differences Between Molarity and Molality

Molarity depends on the volume of the solution, which can change with temperature and pressure because liquids expand or contract. Molality, however, depends on the mass of the solvent, which remains unchanged with temperature and pressure variations.

Title - Situations Favoring Molality Over Molarity

Molality is more accurate than molarity for measuring the concentration of a solution when temperature or pressure varies. This is because molality is based on the mass of the solvent, which does not change with temperature or pressure, unlike volume.

Title - Conclusion

Molality is often preferred in situations where the solution undergoes significant temperature or pressure changes, as it provides a stable measure of concentration.

Key concepts

What Is Molarity?

Molarity, denoted as M, measures the concentration of a solute in a solution. It is calculated based on the volume of the solution. The formula for molarity is given by: Clearly, Clearly, molarity is affected by changes in volume, which in turn are influenced by temperature and pressure variations. For instance, if a solution heats up, the liquid might expand, causing a shift in the concentration measured by molarity. So, when making calculations where temperature or pressure may vary, be cautious because these factors can lead to inaccuracies with molarity.

What Is Molality?

Molality, represented by m, provides a different approach to calculating solute concentration. Unlike molarity, molality focuses on the mass of the solvent. The formula for molality is as follows: The crucial difference here is that mass does not change with temperature or pressure. For example, let’s say you have a solution containing 1 mole of solute in 1 kg of solvent. Whether the temperature rises or falls, the mass of the solvent stays the same, keeping the molality constant. This feature makes molality a more stable measure when dealing with varying temperature and pressure conditions.

Why Molality Over Molarity in Variable Conditions?

Understanding Solution Concentration:
Solution concentration refers to the amount of solute present in a given quantity of solvent or solution. It's a crucial concept in chemistry, helping to describe how 'concentrated' or 'diluted' a solution is.

Situations Favoring Molality:

• Changing Temperature: In scenarios where temperature fluctuates significantly, molality is preferred. Molarity can misrepresent concentration due to the expansion or contraction of the solution volume, but molality remains constant as it is based on mass.
• Pressure Variations: In high-pressure environments, the volume of a solution can compress or expand. Molarity would vary under these conditions, whereas molality, based on the unchanging mass of the solvent, remains reliable.

Example: Think of an aqueous solution used in industrial processes that involves high temperatures. Using molality to measure solute concentration ensures that the readings are accurate and unaffected by the environment’s thermal changes.

In conclusion, while both molarity and molality are essential for discussing solution concentration, molality provides a more accurate measure in environments where temperature or pressure can change. This makes it invaluable for certain scientific and industrial applications.