Counting by Weighing / Lab /

Background

When a chemical reaction takes place, individual atoms and molecules collide and combine or recombine to form new substances. Atoms and molecules are so small that you cannot see them easily, nor can you measure their diameter with a meter stick or measure their mass with a balance. Yet, chemists need to keep track of the number of atoms and molecules, or at least the relationship between the number of atoms and molecules in a chemical reaction. What method has been devised to count the many billions of atoms that are involved in a chemical reaction?

Procedure – Part A

  1. Select 20 smallpaper clips and measure their mass on the balance.
  2. Record the mass and calculate the average mass of 1 paper clip.
  3. Calculate the mass of 100 smallpaper clips.
  4. Add smallpaper clips to the balance until you have a sample, which is approximately equal to this mass.
  5. Count the paper clips you just massed.
  6. Record the actual number of paper clips.
  7. Repeat steps 1-6 using large paper clips instead of small paper clips.
  8. Repeat steps 1-6 using beans instead of paper clips.

Mass of 20 (g) / Average Mass of 1 / Calculated Mass of 100 / Actual Number / Percent Error
Small Paper Clips
Large Paper Clips
Beans

Procedure – Part B

  1. Obtain a sealed envelope of paper clips. A code number and themass of the empty envelopearewritten on the sealed envelope.
  2. Determinethe number of paper clips in the envelope WITHOUT OPENING IT.

Questions:

  1. Describe a method to obtain 1000 smallpaper clipswithout countingthem.
  1. Calculate the expected mass of 12,000 large paper clips. Show yourwork.
  1. Which object did you have the best percent error? Why might that be?
  1. How might counting by weighing be useful in chemistry?
  1. What is a “mole”?
  1. How is a mole similar to a “dozen”?
  1. Describe how you could determine how many atoms or molecules you had in a given sample. What information would youneed to know about the atom or molecule to complete this task?
  1. Chemists actually do count by weighing. Look at the labeled bags on the center table. List the substances and their masses in the space below.
  1. Which substance does there seem to be the most of? ______The least? ______
  1. Actually they all contain the same amount, well at least the same number ofparticles. Explain why they could have the same number of particles, butdifferent masses and volumes.
  1. The bags on the center table all contain 1 moleof the various substances. How many atoms are in each bag?
  1. What would each substance weigh if you had 2 moles of each? How many atoms would be in each bag?
  1. What would each substance weigh if you had 0.5 mole of each? How many atoms would be in each bag?