Calculating Percent by Mass Volume Chem Worksheet 15 2 Answers

Introduction

Calculating percentages by mass and volume is a fundamental skill in chemistry that helps determine the composition of solutions and mixtures. This worksheet focuses on problems that require you to find the percentage of a solute in a solution based on given masses or volumes.

The key concept is understanding that percentage by mass or volume is calculated by dividing the amount of solute by the total amount of solution and then multiplying by 100. This guide will walk you through the process and provide solutions to the specific problems in Worksheet 15-2.

Formula

The general formula for calculating percentage by mass is:

For percentage by volume, the formula is similar but uses volumes instead of masses:

These formulas are essential for solving problems in Worksheet 15-2. The calculator provided on this page implements these formulas to give you quick and accurate results.

Worksheet 15-2 Solutions

Worksheet 15-2 contains several problems that require you to calculate percentages by mass and volume. Below are the solutions to these problems, explained step by step.

Problem 1: Percentage by Mass

A solution is made by dissolving 25 grams of sodium chloride (NaCl) in 100 grams of water. What is the percentage by mass of NaCl in the solution?

Solution:

1. Identify the mass of the solute (NaCl): 25 grams
2. Identify the total mass of the solution: 25 grams (NaCl) + 100 grams (water) = 125 grams
3. Apply the percentage by mass formula:

   (25 g / 125 g) × 100 = 20%

4. The percentage by mass of NaCl in the solution is 20%.

Problem 2: Percentage by Volume

A solution contains 50 milliliters of ethanol in 200 milliliters of water. What is the percentage by volume of ethanol in the solution?

Solution:

1. Identify the volume of the solute (ethanol): 50 mL
2. Identify the total volume of the solution: 50 mL (ethanol) + 200 mL (water) = 250 mL
3. Apply the percentage by volume formula:

   (50 mL / 250 mL) × 100 = 20%

4. The percentage by volume of ethanol in the solution is 20%.

Problem 3: Mixed Units

A solution is made by dissolving 10 grams of sugar in 500 milliliters of water. What is the percentage by mass of sugar in the solution?

Solution:

1. Identify the mass of the solute (sugar): 10 grams
2. Identify the total mass of the solution. Since the volume of water is given, we need to know its density. Water has a density of approximately 1 g/mL, so 500 mL of water weighs 500 grams.
3. Total mass of the solution: 10 g (sugar) + 500 g (water) = 510 g
4. Apply the percentage by mass formula:

   (10 g / 510 g) × 100 ≈ 1.96%

5. The percentage by mass of sugar in the solution is approximately 1.96%.

Common Mistakes

When solving percentage by mass and volume problems, students often make the following mistakes:

• Incorrectly identifying the solute and solvent: It's essential to correctly identify which component is the solute and which is the solvent. Mixing these up will lead to incorrect calculations.
• Forgetting to include the mass or volume of the solvent: The total mass or volume of the solution includes both the solute and the solvent. Forgetting to include the solvent's mass or volume will result in an incorrect percentage.
• Unit conversion errors: When dealing with mixed units (e.g., grams and milliliters), it's crucial to convert all measurements to consistent units before performing calculations.
• Rounding errors: Rounding intermediate results too early can lead to significant errors in the final percentage. It's best to keep more decimal places during calculations and round only the final answer.