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LAB in Percent Composition, Introduction, Percent Composition, Scientific Method. Last updated December 19, 2022.


In this lab, students will develop a theory of how to calculate percent composition using household items such as lentils and beans.

Grade Level

Middle school

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

  • MS-PS1-5: Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.
  • Scientific and Engineering Practices:
    • Planning and Carrying Out Investigations
    • Using Mathematics and Computational Thinking


By the end of this lab, students will

  • Have a better understanding of mass percent.

Chemistry Topics

This labsupports students’ understanding of

  • Percent composition


Teacher Preparation: 15 minutes

Lesson: 50 minutes (one class period)


For each group:

  • Cup of legumes (a mixture of beans and lentils)
  • Balance
  • Calculator


  • No specific safety precautions need to be followed.

Teacher Notes

  • I thought it was easy to do with the students, no safety precautions necessary, and it's a good activity that helps them develop scientific inquiry skills.

For the Student



The percent composition of a component in a compound is the percent of the total mass of the compound that is from a specific component. Percent compositions, or mass percent, exist in many aspects of daily life. For example, FDA requires that nutrition labels list food as a percent of the recommended daily value. On all nutrition labels, a known serving size is broken down in five categories: total fat, cholesterol, sodium, total carbohydrate, and protein. The mass of each category that is in the food, except protein, is converted to a percent of daily value—what part of a person’s total suggested serving is in the one serving size in the container. These calculations are based on the masses of each category recommended per day per person who has a 2,000 Calories diet.

For a chemistry example, consider the compound NaCl. You know that there is one atom of sodium for every one atom of chlorine, but that doesn’t mean that 50% of the compound by mass is sodium. You need to know the mass of sodium and the mass of chlorine to make that calculation.


How is percent composition calculated?

Prelab Questions

Obtain a sample of beans and lentils. Talk with your lab partner and answer the following questions:

  1. What information will you need to calculate the mass percent of each component?
  2. How will you get that information?
  3. What materials do you need? Why?
  4. Your sample contains a mixture of beans and lentils. Predict which you think has the higher mass percent. Why?


Create a procedure for your experiment and write it below. Why do you think that this procedure will be appropriate?


Create a data table to record the information that you gather. Make sure to label all numbers with correct units.


What is the mass percent of beans in your mixture? What is the mass percent of lentils? Show all work (including sig figs and labels).


  1. Which substance had the highest mass percent? Was your prediction correct?
  2. Would changing the amount in the original sample change the results? Why?
  3. What are the possible sources of error in your lab?
  4. Would you change your procedure if you were to repeat the experiment? Why?