The percent composition of a solution, often referred to as percent composition by mass, is a way to express the concentration of a solute within a solution. It is defined based on the masses of the components.
According to the definition, Percent Composition by Mass is the mass of the solute divided by the mass of the solution (mass of the solute plus mass of the solvent), multiplied by 100. This formula tells you what percentage of the total mass of the solution comes from the solute.
Understanding the Components
To grasp percent composition, it's essential to understand the key terms:
- Solute: The substance that is dissolved in the solvent. It is usually present in a smaller amount.
- Solvent: The substance that dissolves the solute. It is usually present in a larger amount. Water is a very common solvent.
- Solution: The homogeneous mixture formed when the solute is dissolved in the solvent.
The Formula
The formula for calculating percent composition by mass is:
Percent Composition by Mass = (Mass of Solute / Mass of Solution) × 100%
Since the mass of the solution is the combined mass of the solute and the solvent, the formula can also be written as:
Percent Composition by Mass = (Mass of Solute / (Mass of Solute + Mass of Solvent)) × 100%
Why is Percent Composition Used?
Using percent composition by mass is a common and straightforward way to describe the concentration of a solution. It's particularly useful because mass is easily measured in a laboratory, and it's not affected by temperature changes, unlike volume. This makes it a reliable measure for comparing concentrations.
Practical applications include:
- Chemistry: Preparing solutions of specific concentrations for experiments.
- Pharmacy: Formulating medications with precise ingredient amounts.
- Manufacturing: Controlling the composition of products like cleaning supplies, fertilizers, or foods.
Example Calculation
Imagine you dissolve 10 grams of salt (solute) in 90 grams of water (solvent).
- Identify the Mass of Solute: 10 grams
- Identify the Mass of Solvent: 90 grams
- Calculate the Mass of Solution: Mass of Solute + Mass of Solvent = 10 g + 90 g = 100 grams
- Apply the Formula:
Percent Composition by Mass = (10 g / 100 g) × 100%
Percent Composition by Mass = 0.1 × 100%
Percent Composition by Mass = 10%
So, the solution is a 10% salt solution by mass.
This simple metric provides a clear representation of the relative amount of dissolved substance in a liquid mixture.
