Counting Molecules and Atoms: Number of Molecules of Chalk in Your Signature (page 2)

Chemists often work with large numbers of small particles. To make counting easier, they use a unit called a mole. One mole of anything is equal to 6.02 × 10²³. Chalk is calcium carbonate: CaCO₃. One mole of calcium carbonate has a molar mass of 100 grams. Using this information, you can mathematically calculate how many molecules of chalk you use when signing your name on the board.

Materials

Triple beam or electronic balance; Access to a chalkboard; Piece of chalk; Calculator

Activity

1. Use the balance to weigh and record the mass of the piece of chalk.
2. Sign your full name on the chalkboard.
3. Reweigh the piece of chalk and record the mass.
4. Subtract the new mass from the original mass to get the number of grams of calcium carbonate you used to write your name.
5. Convert the grams of chalk to moles of chalk by dividing the grams of chalk used by 100 grams, the molar mass of calcium carbonate.
6. Convert the number of moles of chalk used to the number of molecules of chalk used by multiplying the number of moles by 6.02 × 10²³. This tells you the number of molecules of calcium carbonate required to write your name.
7. If time allows, compare your calculations with your classmates' results.

Follow-Up Questions

1. How many moles of calcium carbonate did you use to sign your name?
2. How many molecules of calcium carbonate did you use to sign your name?

Answers

1. Answers will vary.
2. Answers will vary.

Extension

If you want to determine the number of atoms of calcium carbonate you used when signing your name, multiply the number of molecules by 5. What number did you get? Why do you think you had to multiply by 5 to get this?