Summary

In this lesson, students will better understand the mole concept by using chalk to write their name on a sidewalk and finding out the composition of chalk through research to calculate the moles of chalk in their names.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the following scientific and engineering practices:

• Scientific and Engineering Practices:
  • Asking Questions and Defining Problems
  • Using Mathematics and Computational Thinking
  • Planning and Carrying Out Investigations

Objectives

By the end of this lesson, students should be able to:

• Design and implement a procedure to determine the number of moles and formula units of chalk used in writing their names.

Chemistry Topics

This lesson supports students’ understanding of:

• Mole concept
• Dimensional analysis

Time

Teacher Preparation: 5 minutes

Lesson: 25 minutes

Materials

• Sidewalk chalk
• Balance (the more precise, the better)
• 3x5 note card for each student

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• Students should be introduced to the concept of a mole and molar mass prior to this lesson.
• Since chalk is an ionic compound, they will be determining the number of formula units (rather than molecules), so you should be sure to differentiate between these terms before you start.
• I start the exercise by showing the glass a glass of water and asking them to determine what they would need to know to calculate the number of moles and the number of molecules of water in the glass. Once I am convinced that they understand the need to find the mass of the water (along with its molar mass from the periodic table), I ask them if I gave them some chalk and they used it to write, could they determine the moles of chalk that was used. I do not give them the molecular formula for chalk (CaCO₃, MM = 100.09 g/mol) but I do allow them to use their smart phones, tablets, or laptops to do the research.
  • If students are hesitant to engage in a full-class discussion, try having them brainstorm ideas with a partner or small group first before presenting their ideas to the whole class.
• I leave this activity open ended so students think their way
  through this exercise. When students are thinking through the glass of water question before starting with the chalk, make sure they come up with a specific list of all the measurements they would need as well as what equipment they would use to take those measurements. Then congratulate them for designing an experiment all on their own! They will likely be exited to know that they just
  did what researchers do (albeit on a fairly simple topic, but understanding the process is a big first step) when they start a new research project. Then allow them to apply this to the chalk question on their own or with a partner.
• Next, I hand out 3x5 cards for students to record the following, with the proper units:
  • Any data they record, clearly labeled and including appropriate units (they should record the mass of chalk before and after writing their names)
  • Calculation of moles used to write their name, showing work and units
    • As part of this, they will need to research the main chemical compound in chalk (calcium carbonate) and determine its formula (CaCO₃) and molar mass (100.09 g/mol)
  • Calculation of formula units of chalk used to write their name, showing work and units
• I set out a tub of sidewalk chalk and have students go outside to write their name on the sidewalk. (This could also be done on a chalkboard or piece of paper if weather or lack of outdoor space is a problem.) They then calculate on their notecard how many moles of chalk and how many formula units of chalk were used to write their name.
  • You could have them write the number of formula units out long-form in addition to in scientific to emphasize just how many particles they used!
• Students who have short names or write their names small may not notice a difference in mass. This is a good place for them to practice troubleshooting and analyzing/solving a problem with their data collection! If this happens, encourage students to think about why they don’t see a difference in mass (balance isn’t precise enough to notice the small difference in mass) and have them try again, writing their name bigger/more decoratively.
  • Even though this is a good learning moment, it may still be helpful to have as precise a balance you can get (at least 2 but preferably 3 decimal places)
• As an extension activity, you could have the students calculate how many times they could write their names with their piece of chalk before they would run out of chalk.
• Students like everything about this activity: writing their names on the sidewalk, figuring out what chalk is, finding its molar mass, and finding out the huge number of formula units used just to spell out their name.
• The AACT resource library also contains a similar activity using crayons (paraffin wax) and a coloring page called “Can You Color a Mole?” if you want to avoid the distinction between formula units used to describe ionic compounds like chalk and molecules used to describe molecular compounds like water and paraffin wax.

Sample student responses:

Mass of chalk before writing name: 20.095 g   Mass of chalk after writing name: 19.776 g

Mass of chalk used to write name: 20.095 g – 19.776 g = 0.319 g

Chemical in chalk: Calcium carbonate, CaCO₃

Molar Mass: 40.08 g/mol + 12.01 g/mol + 3(16.00 g/mol) = 100.09 g/mol

Moles of chalk:  0.319 g x (1 mol / 100.09 g) = 0.00319 mol

Formula units of chalk: 0.00319 mol x (6.02 form. un. / 1 mol) = 1.92 x 10²¹ formula units CaCO₃