Calculating Percent by Mass/volume Chem Worksheet 15-2

This comprehensive guide explains how to calculate percentages by mass and volume in chemistry, with a focus on the concepts covered in worksheet 15-2. You'll learn the key formulas, see practical examples, and get access to a downloadable practice worksheet.

Introduction

Calculating percentages by mass and volume is a fundamental skill in chemistry that helps you determine the composition of substances and solutions. This guide focuses on the concepts covered in worksheet 15-2, which typically includes problems involving the percentage of a solute in a solution, percentage composition of compounds, and related calculations.

The key concepts you'll learn include:

Calculating percentage by mass
Calculating percentage by volume
Understanding the difference between mass and volume percentages
Applying these calculations to real-world chemistry problems

By the end of this guide, you'll be able to confidently solve percentage composition problems similar to those in worksheet 15-2.

Key Formulas

The two main formulas for calculating percentages in chemistry are:

Percentage by Mass

Formula: % by mass = (mass of component / total mass of sample) × 100

Where:

mass of component = mass of the specific part you're interested in
total mass of sample = mass of the entire sample

This formula is used when you're dealing with the mass of components in a sample.

Percentage by Volume

Formula: % by volume = (volume of component / total volume of solution) × 100

Where:

volume of component = volume of the specific part you're interested in
total volume of solution = volume of the entire solution

This formula is used when you're dealing with the volume of components in a solution.

Note: Remember that percentage by mass and percentage by volume are different concepts. Always check which type of percentage is being asked for in the problem.

Worked Examples

Let's look at some practical examples to see how these formulas work in real chemistry problems.

Example 1: Percentage by Mass

Problem: A sample of salt (NaCl) has a mass of 5.85 g. The total mass of the sample is 100 g. What is the percentage by mass of NaCl in the sample?

Solution:

Identify the mass of the component (NaCl) = 5.85 g
Identify the total mass of the sample = 100 g
Apply the percentage by mass formula:
% by mass = (5.85 g / 100 g) × 100 = 5.85%

Answer: The percentage by mass of NaCl in the sample is 5.85%.

Example 2: Percentage by Volume

Problem: A solution contains 25 mL of ethanol in a total volume of 500 mL. What is the percentage by volume of ethanol in the solution?

Solution:

Identify the volume of the component (ethanol) = 25 mL
Identify the total volume of the solution = 500 mL
Apply the percentage by volume formula:
% by volume = (25 mL / 500 mL) × 100 = 5%

Answer: The percentage by volume of ethanol in the solution is 5%.

Practice Worksheet

To reinforce your understanding, try solving these practice problems similar to those in worksheet 15-2. Answers are provided at the end.

Problem 1

A sample of sugar (C₁₂H₂₂O₁₁) has a mass of 10.0 g. The total mass of the sample is 200 g. What is the percentage by mass of sugar in the sample?

Problem 2

A solution contains 50 mL of acetic acid in a total volume of 1000 mL. What is the percentage by volume of acetic acid in the solution?

Problem 3

A compound has two components with masses of 3.5 g and 6.5 g. What is the percentage by mass of each component in the compound?

Answers

5.0%
5.0%
35% and 65%

Tip: Always double-check your calculations and units to ensure accuracy. If you're unsure about a problem, try working through it step by step.

Frequently Asked Questions

What is the difference between percentage by mass and percentage by volume?: Percentage by mass refers to the ratio of the mass of a component to the total mass of the sample, while percentage by volume refers to the ratio of the volume of a component to the total volume of the solution. They measure different properties of the sample or solution.
When should I use percentage by mass versus percentage by volume?: Use percentage by mass when dealing with the mass of components in a sample, and use percentage by volume when dealing with the volume of components in a solution. Always check the problem statement to determine which type of percentage is being asked for.
How do I handle problems with multiple components?: For problems with multiple components, you can calculate the percentage of each component separately using the appropriate formula. The sum of all percentages should be 100% if you're considering all components in the sample or solution.
What units should I use for mass and volume measurements?: For mass, use grams (g) or kilograms (kg), and for volume, use milliliters (mL) or liters (L). Ensure all measurements are in consistent units before performing calculations.
How can I check if my percentage calculation is correct?: To verify your calculation, you can reverse the process. For example, if you calculated 10% by mass, you can multiply the total mass by 0.10 to see if you get the mass of the component. This can help you catch any calculation errors.