π What is Percent by Mass?
Percent by mass is a way to express the concentration of a substance in a mixture or solution. It represents the mass of the solute (the substance being dissolved) divided by the total mass of the solution, multiplied by 100 to express it as a percentage.
π§ͺ History and Background
The concept of expressing concentrations as percentages has been used in chemistry for a long time. It provides a straightforward way to communicate the composition of mixtures, especially in industrial and analytical contexts.
π Key Principles
- βοΈ Definition: Percent by mass is defined as: $\text{Percent by Mass} = \frac{\text{Mass of Solute}}{\text{Mass of Solution}} \times 100$
- β Mass of Solution: The mass of the solution is the sum of the masses of the solute and the solvent (the substance doing the dissolving): $\text{Mass of Solution} = \text{Mass of Solute} + \text{Mass of Solvent}$
- π― Percentage: The result is multiplied by 100 to express the concentration as a percentage.
βοΈ Calculating Percent by Mass: Step-by-Step
- π Step 1: Determine the mass of the solute.
- π§ Step 2: Determine the mass of the solvent.
- β Step 3: Calculate the total mass of the solution by adding the mass of the solute and the mass of the solvent.
- β Step 4: Divide the mass of the solute by the mass of the solution.
- βοΈ Step 5: Multiply the result by 100 to get the percent by mass.
π Real-World Examples
- π Saline Solution: A saline solution might be 0.9% NaCl by mass, meaning there are 0.9 grams of NaCl in 100 grams of solution.
- π· Vinegar: Vinegar is typically 5% acetic acid by mass.
- π§ͺ Hydrogen Peroxide: Household hydrogen peroxide is often a 3% solution.
βοΈ Example Calculation
Problem: What is the percent by mass of a solution made by dissolving 30.0 g of NaCl in 150.0 g of water?
Solution:
- π Mass of Solute (NaCl): 30.0 g
- π§ Mass of Solvent (Water): 150.0 g
- β Mass of Solution: 30.0 g + 150.0 g = 180.0 g
- β Percent by Mass: $\frac{30.0 \text{ g}}{180.0 \text{ g}} \times 100 = 16.7\%$
π‘ Conclusion
Percent by mass is a fundamental concept in chemistry for expressing the concentration of solutions. It provides a clear and practical way to understand the composition of mixtures, making it essential in various fields from laboratory research to industrial applications. Understanding this concept allows for accurate preparation and analysis of chemical solutions.