An experiment with "atomic mass"

The purpose of the following experiment is to get you familiar with how to measure relative masses. There are no significant dangers in this lab, but do use common sense and be careful at all times. 

Procedures 

1. Find the mass of three labelled styrofoam cups, or beakers.
2. Time and record (in seconds) how long it takes you to count equal number of grains of dried rice, of dried peas, and of dried beans (or other dried grains you may be given by your teacher), into each of the labelled cups. All the rice goes in one cup, all the peas in another, and so on. The actual number you use doesn't matter much (within reason), so you'll probably want between 25 to 50 seeds, but you must have the same number of each. Try and pick individual seeds that look pretty close in size, and discard any that are broken. Note: your teacher may assign you a number of seeds to use, so that all the groups have a different number.
3. Find the mass of the seeds in each cup. Subtract the empty cup's mass to get the mass of the seeds alone.
4. Complete the following data table. To calculate the Relative Mass column, identify the lightest of all seeds (it should probably be rice). Divide the Mass of Seeds for each type by this lightest mass, and enter the result in the Relative Mass column. The lightest seed will have a relative mass of 1, and the others will be heavier.
5. The definition of "Atomic Mass" is that it is the relative mass of the atoms. Compare your atomic mass with other students who did not use the same number of seeds as you.
6. Add seeds of the lightest grain one by one to the balance, until you have as close to 1 g as you can get. Count them. Since the atomic mass of the lightest grain is 1, this is the number you need to fill in the last column of the table. Repeat this procedure for the other grains, except each time count the number of seeds needed to make up the "atomic mass" in grams.

Conclusion 

1. How long would it have taken you to count 1000 seeds? How about a million of them? Do some research and find out the size of Avogadro's number. How long would it have taken you to count out Avogadro's number of seeds?
2. How did your atomic mass compare with students that used different numbers of seeds? Explain your finding.
3. How many grains of rice did it take for you to have a mass equal to the relative mass of rice measured in grams? How about for the peas? The beans? Compare your answers with other students to eliminate random errors, and see what you can conclude.
4. In what way is your answer to the previous question very similar to the idea of a mole in Chemistry?

Variations on a theme: 

1. If you live in an agricultural area, use locally produced grains.
2. Mix all the grains together in one container and have students "mine" and separate them. You can also mix them in various quantities to get across the concept of natural abundances of elements.

Copyright © 1998 - 2008 David Dice