Molality Calculator

How to Calculate Molality

Molality is a measure of the concentration of a solution, expressed as the number of moles of solute per kilogram of solvent. It's an important concept in chemistry, particularly useful when dealing with colligative properties of solutions.

What is the Formula?

The formula for molality is:

\[ \text{Molality} = \frac{\text{moles of solute}}{\text{kilograms of solvent}} \]

Where:

• Molality is expressed in mol/kg
• Moles of solute is the amount of dissolved substance
• Kilograms of solvent is the mass of the dissolving medium

What are the calculation steps?

1. Determine the number of moles of solute
2. Measure the mass of the solvent in kilograms
3. Divide the moles of solute by the kilograms of solvent

Example Calculation

Let's calculate the molality of a solution containing 2.5 moles of NaCl (table salt) dissolved in 1.5 kg of water:

Given:

• Moles of solute (NaCl) = 2.5 mol
• Mass of solvent (water) = 1.5 kg

Step 1: Identify the formula

Molality = moles of solute / kilograms of solvent

Step 2: Substitute the values

Molality = 2.5 mol / 1.5 kg

Step 3: Solve the equation

Molality = 1.67 mol/kg

Therefore, the molality of the NaCl solution is 1.67 mol/kg.

Diagram of Molality

The following diagram illustrates the concept of molality:

This diagram represents a molal solution. The green circle symbolizes the solute (in moles) dissolved in 1 kilogram of solvent (blue square). Molality is independent of temperature because it's defined using mass rather than volume.