Counting Stuff by Mass...or How Many Candy Corns Are in That Fishbowl?

Apr 20 2011

Source: John Tracey

Summary Information

Keywords:

atomic mass, molar mass, moles, stoichiometry, atomic weight, measurement

Topic:

Measurement

Target Grade (Ages):

Grade 8 (Ages 13-14)

Estimated Time Required:

less than 1 hour

Diversity Indicators:

This lesson is appropriate for kinesthetic and visual learners.

Tab Wrapper

LESSON

Goals and Prerequisites

Goals:

To establish understanding about atomic weight and molar mass.
To introduce students to the concept of counting large numbers of objects using only the mass of the objects.
Relate the students counting the objects in the lab to chemists counting atoms and molecules by using only the mass of the sample.

Prerequisites:

Students should:

Be able to use a triple beam balance to mass a sample.
Be able to determine the atomic mass of an element using the periodic table. Be able to calculate the molar mass of a compound.

Instructional Objective(s)

At the end of the lesson students should be able to:

Articulate the concept of atomic weight and molar mass
Explain how chemists use the mass of a sample to determine the number of formula units in the sample.

Instructional Resources

Background:

Each group of students will relate the time and effort required to count out by hand a large number of small objects to the time and effort required to count the same sample using only the mass of the sample. The group will then be asked to determine the number of objects in a different sample as a check in exercise. The students can then apply what they have learned in the lab to solve the challenge question that was posed at the beginning of the lab.

How does this relate to atoms, atomic weight, molar mass and moles?

Each atom has an average atomic weight. This is the decimal number on the periodic table. The average mass of a paper clip is like the atomic weight of an atom.

To get the average mass of a compound, chemists add up the atomic weights of the individual atoms. For example, water has a formula of H₂O. One mole of water has the mass of the mass of two hydrogen atoms and one oxygen atom.

Water = 18 g/mole = 1 Hhydrogen (1 g/mole) + 1 Hydrogen (1 g/mole) + 1 Oxygen (16 g/mole)

I can tell how many moles of water I have by determining the mass of the water and then dividing by the mass of one mole. Therefore 180 grams of water would be 10 moles of water.

180 grams / 18 grams/mole

There are many tutorials available on the internet. One you can check out is:

http://www.occc.edu/kmbailey/Chem1115Tutorials/Stoichiometry_Molar_Mass.htm

An interesting site that is also worth checking out is:

http://www.chemheritage.org/

Before implementing this lesson, you should figure out the average mass of one large paper clip. You will provide this number to students in Step 9 of the procedure.

Materials:

Sample Student/Class Lab Sheet
About 100 small paperclips per group
About 100 large paperclips per group
Any small and larger object can be used. (For example, copper BBs and plastic airsoft pellets)
Balance
Stop watch

Sample Student/Class Lab Sheet (doc)

Procedure:

This lesson can be completed in one extended block.

The lesson starts with a challenge question which the students will answer at the end of the lab as a check out exercise.

Challenge question

Your favorite game store is running a contest where you must guess the number of Acme brand candy corn pieces that are in a sealed fish bowl. You only have 20 minutes before the contest ends, and the owner says you can do anything except open the bowl. The store next door is a general store that carries normal household items and the same fish bowl and candy corn. Create a procedure that lets you determine the number of candy corn in the bowl without counting them manually. (There just isn’t enough time!)

Procedure

Please see Sample Student/Class Lab Sheet.
Count out exactly 100 small paper clips. Using a stop watch record the amount of time this takes. Record this time on your data chart in the appropriate column. Students will notice this is a time consuming task.
Use the Triple Beam Balance to find the mass of the 100 small paper clips you just counted out, and record this mass on your data chart in the appropriate column. Remember to record the mass of the empty plastic tray.
Find the mass of exactly 20 small paper clips. We are using 20 paper clips because it is a small enough number that it isn't quite as time consuming to count out as 100 paper clips, but it is still a large enough number to reduce error. Use this number to determine the average mass of one small paper clip.

Average mass of small paper clip = mass of 20 small paper clips – mass of plastic tray / 20

Multiply the average mass of 1 paper clip you calculated in step 4 by 100 to calculate the mass of 100 small paper clips. This will be referred to as the calculated mass. Record this number on your data chart in the appropriate column.
Calculate the % error:

(Calculated mass of small paper clips – Actual mass of small paper clips / Calculated mass of small paper clips) X 100.

Record this number on your data chart in the appropriate column. Students will notice that the actual mass is very close to the calculated mass and that it was much quicker to calculate the mass than to count out 100 paper clips and weigh them.

Measure a mass of small paper clips equal to the calculated mass. Set the balance to the mass of the plastic tray + the calculated mass of 100 small paper clips. Slowly add small paper clips till the arm of the balance starts to move, then add or remove small paper clips one by one till the balance reads zero. Use a stop watch record the amount of time this takes. Record this time on your data chart in the appropriate column. Again, students will notice this is much faster than counting out 100 paper clips one by one.
Count out the number of small paper clips you actually obtained in step 7 and compare this to 100. Calculate the % error:

(100 – Actual number of small paper clips / 100) X 100.

Record these numbers on your lab sheet in the appropriate column. Students will notice that this quicker method is still fairly accurate.

You will now be given an unknown number of large paper clips. Use the method of ‘counting by mass’ to determine the number of large paper clips you have been given. The average mass of a large paper clip will be provided for you by your teacher. Students can weigh the large paper clips and then divide their result by the average mass you have provided to determine the number of large paper clips. Now actually count out the number of paper clips. Was your calculation close to the actual number of paper clips?
Clean up your lab table. Put all the small paper clips in the glass bottle, and put the bottle and the stopwatch and the plastic tray into the large bowl.
Return to you seat for class discussion.

Sample Student/Class Lab Sheet (doc)

Extensions

Given the time and materials, a good tie in to a robotics program might be for the students to design and execute a robot to count out metal BBs or a simular product.

Assessments and Rubrics

Now ask the students to answer the challenge question and turn the answer in as an "exit ticket."

Challenge question

Your favorite game store is running a contest where you must guess the number of Acme brand candy corn pieces that are in a sealed fish bowl. You only have 20 minutes before the contest ends and the owner says you can do anything except open the bowl. The store next door is a general store that carries normal household items and the same fish bowl and candy corn. Create a procedure that lets you determine the number of candy corn in the bowl without counting then manually. (There just isn’t enough time!)

Sample Lab Report Questions

Compare the time it took to count out the small paper clips compared to the time it took to get a number of small paper clips by mass.
Based on your percent error calculations, how accurate is counting small paper clips by mass?
Why is counting atoms by mass like counting small paper clips by mass?

Credits

John Tracey, teacher of physics and chemistry, currently at Mount Olive High School

jtracey@mtoliveboe.org

RESOURCES

Original Resources

Sample Student/Class Lab Sheet (doc)

Contributed Resources

Contribute a Resource

Assessment Strategies and Lesson Revision

Revised lesson for intro high school chemistry class as well as formative and summative assessment strategies

Sample Student Work for Revised Lesson

Student work with teacher feedback.

STANDARDS

Click here to add Standards

NJ Core Content Curriculum Content Standards

Standard Type: State New Jersey

New Jersey Core Content Curriculum Content Standards

5.1 Science Practices

All students will understand that science is both a body of knowledge and an evidence-based, model-building enterprise that continually extends, refines, and revises knowledge. The four Science Practices strands encompass the knowledge and reasoning skills that students must acquire to be proficient in science.

5.1.12.A.3 Use scientific principles and theories to build and refine standards for data collection, posing controls, and presenting evidence.
5.1.12.B.1 Design investigations, collect evidence, analyze data, and evaluate evidence to determine measures of central tendencies, causal/correlational relationships, and anomalous data.
5.1.12.B.3 Revise predictions and explanations using evidence, and connect explanations/arguments to established scientific knowledge, models, and theories.
5.1.12.B.4 Develop quality controls to examine data sets and to examine evidence as a means of generating and reviewing explanations.
5.1.12.C.2 Use data representations and new models to revise predictions and explanations.

Your rating

Average: 5 (1 vote)

Comments

mfrank78

Permalink

April 27, 2011 - 8:14pm

This will be a great physics

This will be a great physics lesson to use.

Karen2012

Elmhurst, New York, United States

Permalink

May 19, 2012 - 1:37pm

Counting stuff by mass modified lesson

The opening assessment is presented in a very non threatening way for students and allows students to engage in understanding. the idea of the mole is not always easy for students to understand so the assessment strategies is a great guided way to explore this concept!

asc006

South River, New Jersey, United States

Permalink

May 19, 2012 - 2:08pm

Assessment Strategies and Lesson Revision

I know that I will be using some version of this assessment in my chemistry class. I think this is a great way to get students thinking about the relationship for mass and moles and why atoms are often counted using the mole. Great assessment - thanks!

Teachers TryScience

Languages

Main menu

Keywords:

Target Grade (Ages):

Estimated Time Required:

Diversity Indicators:

Tab Wrapper

Goals:

Prerequisites:

Background:

Materials:

Procedure:

Original Resources

Contributed Resources

Comments

This will be a great physics

Counting stuff by mass modified lesson

Assessment Strategies and Lesson Revision

Teachers TryScience

User Login Custom

Languages

Main menu

Search form

Keywords:

Target Grade (Ages):

Estimated Time Required:

Diversity Indicators:

Tab Wrapper

Goals:

Prerequisites:

Background:

Materials:

Procedure:

Original Resources

Contributed Resources

Comments

This will be a great physics

Counting stuff by mass modified lesson

Assessment Strategies and Lesson Revision