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AVERAGE ATOMIC MASS LAB

BACKGROUND

All of the elements on the Periodic Chart exist in at least two isotopic forms. Isotopes are atoms whose atomic numbers are the same but whose mass numbers are different due to differing numbers of neutrons present in each isotope. The atomic mass, which is shown on the Periodic Chart for that element, is actually an average of all the isotopes of the element, weighted by the percentage of the abundance in which they occur. It should be noted that different isotopes of an element are not found in different places. Every sample of that element contains all of the isotopes in fixed percentages, no matter the element is found.

NEWS FLASH!!! A new element has just been discovered. Farm CityUSA. Nuclear chemists, performing basic research on food products have discovered what is believed to be a new element. The researchers have tentatively named this element Beanium (bean' ee umm) derived from the protein nodules we put into our chili recipes.

Further research of the new element will be conducted in more suitable surroundings, namely laboratories in a nearby high school. STUDENT EXCITEMENT REGARDING THIS DISCOVERY IS RUNNING AT FEVER PITCH! Many chemistry students have generously volunteered their time and expertise to help with the follow-up experiments involving the new element.

A reliable source was overheard to say that the first follow-up experiments conducted in the high schools will determine how many isotopes of this element exist, the mass of each of these isotopes, the percentage of abundance of each isotope and the average atomic mass of the new element. One unique property of Beanium should make these experiments particularly easy--unlike normal atoms, Beanium atoms are very large. They can be easily seen, and the different isotopes can be recognized and sorted by hand. Scientists are expecting a complete, comprehensive summary of this new element within two days, including pictures and collected data tables. This is the most exciting discovery in chemistry of the century!!!

OBJECTIVE

To identify the number of isotopes of Beanium, determine the mass of each of the isotopes, find the percent of abundance of each isotope and calculate the average atomic mass of the new element Beanium.

EQUIPMENT

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balance

sample of Beanium

plastic counting plate

calculator

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PROCEDURE

  1. Sort each of the different isotopes (beans) into piles.
  2. Count each pile of isotopes (beans).
  3. Find the mass of the plastic counting plate.
  4. Add each isotope sample (all of the beans of one type) to the plastic counting plate and find the total mass.

REMEMBER: All mass measurements should be recorded to the hundredths place!

DATA

Write your data in a clear and organized manner, preferably in a table.

CALCULATIONS

1. Show one sample of finding the mass of each (individual) isotope – take the total mass and divide by the number of that isotope to get the mass of a single bean. (4 sig figs)

2. Show one sample of finding the abundance of each isotope – take the number of each isotope and divide it by the sum of ALL of the isotopes. (4 sig figs)

3. Show ALL of the work for finding the average atomic mass (4 sig figs)

RESULTS & CONCLUSION

Written in complete sentences in paragraph form

1. Describe what you did (relate it back to the objective) and what you found (calculated average atomic mass)

2. Discuss sources of error

a. What difference would it make if you miscounted a bean with a small abundance vs. a bean with a large abundance?

b. What difference would it make if your total mass was wrong for a bean with a small abundance vs. a bean with a large abundance?

c. Should all groups in the class get the same average atomic mass? Why or why not? If we were working with real atoms would all the groups in the class get the same average atomic mass?