Question

Differentiate between molarity and molality.

Short answer

Molarity measures concentration as moles per liter of solution, while molality measures it as moles per kilogram of solvent.

Step-by-step solution

Define Molarity

Molarity (M) is defined as the number of moles of solute per liter of solution. It is a measure of the concentration of a solute in a solution and is expressed as moles per liter (mol/L). The formula to calculate molarity is: \( M = \frac{n}{V} \), where \( n \) is the number of moles of the solute, and \( V \) is the volume of the solution in liters.

Define Molality

Molality (m) is defined as the number of moles of solute per kilogram of solvent. It is expressed as moles per kilogram (mol/kg) and is not dependent on the volume of the solution, which can change with temperature. The formula to calculate molality is: \( m = \frac{n}{m_{solvent}} \), where \( n \) is the number of moles of solute, and \( m_{solvent} \) is the mass of the solvent in kilograms.

Highlight the Differences

The primary difference between molarity and molality is how they measure concentration. Molarity is dependent on the volume of the solution, whereas molality depends on the mass of the solvent. This makes molality a useful concentration measure because it is unaffected by changes in temperature, unlike molarity, which can vary with temperature changes due to the expansion or contraction of the solution.

Key concepts

Solute Concentration

Understanding solute concentration is crucial when studying solutions in chemistry. Solute concentration refers to the amount of solute—that is, the substance being dissolved—in a given quantity of solvent or solution. Concentration is significant because it tells us how much solute is present in a solution, affecting the solution's properties and reactions.
In our daily lives, understanding concentration can help explain why some drinks are sweeter than others, or why certain medications are more potent.
There are different ways to express concentration, such as percentage, molarity, and molality, each having its unique applications and units. These measurements help chemists and scientists quantify and understand how solutions behave under various conditions.

Molarity Calculation

Molarity is a common and practical way to express solute concentration in a solution. It is defined as the number of moles of solute per liter of solution. When calculating molarity, we use the formula:
\[ M = \frac{n}{V} \]
Where:

• \( M \) is the molarity.
• \( n \) is the number of moles of solute.
• \( V \) is the volume of the solution in liters.

For example, if you have a 1-liter solution with 2 moles of salt dissolved in it, the molarity would be 2 mol/L.
Molarity is temperature-dependent. As temperature increases, the volume of the solution can also increase, leading to a decrease in molarity as the solution becomes more dilute. This is important to consider in experiments sensitive to temperature changes.

Molality Calculation

Molality offers an alternative way to measure concentration that is particularly useful when temperature changes are a factor. Molality is the number of moles of solute per kilogram of solvent. The formula for molality is:
\[ m = \frac{n}{m_{solvent}} \]
Where:

• \( m \) is the molality.
• \( n \) is the number of moles of solute.
• \( m_{solvent} \) is the mass of the solvent in kilograms.

For example, if you dissolve 2 moles of a substance in 1 kilogram of water, the molality would be 2 mol/kg.
A key advantage of molality is that it does not change with temperature, since it is based on the mass of the solvent, which remains constant regardless of thermal changes. This makes molality ideal for calculations in situations where precise concentration is essential across varying environmental conditions.