ChemLab #3 - Isotope Abundance and Molar Mass
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Introduction: Most elements consist of mixtures of isotopes. Their atomic weights are derived using the concept of weighted averages. In this lab, you will simulate one way scientists determine the relative amounts of isotopes present in a sample of an element.

An imaginary element "pennium" consists of a mixture of pre-1982 and post-1982 pennies. These pennies have different compositions, therefore different masses. They represent the two isotopes of "pennium".     (Lab Key)

You will have a sealed envelope containing a mixture of the two "isotopes". The envelope could contain any combination of the two. Your task is to determine the isotope composition of pennium without opening the envelope.

  • The envelope contains an unknown mixture of 10 pennies. Let x = the number of pre-1982 pennies and 10 - x = the number of post-1982 pennies.
  • The mass of ALL pre-1982 pennies is equal to the number of pre-1982 pennies (x), multiplied by the mass of one pre-1982 penny.
  • The mass of ALL post-1982 pennies is equal to the number of post-1982 pennies (10 - x), times the mass of one post-1982 penny.
  • This relationship is expressed as: Total mass of pennies = (x) (mass of 1 pre-1982 penny) + (10 - x) (mass of 1 post-1982 penny)
Student Materials:     1 envelope, 1 pre-1982 penny, 1 post-1982 penny
Experimental Design:
  1. Find the mass of the empty envelope to the nearest 0.1g.
  2. Give the envelope to your facilitator. He will put 10 pennies in it and give it back to you sealed.
  3. Find the mass of the envelope with pennies to the nearest 0.1g.
  4. Find the mass of 1 pre-1982 penny and 1 post-1982 penny seperately, to the nearest 0.1g.
     
    Data Table
    Mass of empty envelope   ________ g
    Mass of envelope with 10 pennies   ________ g
    Total mass of pennies in envelope   ________ g
    Mass of 1 pre-1982 penny   ________ g
    Mass of 1 post-1982 penny   ________ g
    		 
Calculations: (All calculations MUST be shown in the white space above right.)
  1. Determine the total mass of the 10 pennies in the envelope.
  2. Calculate the value of x (the number of pre-1982 pennies) and 10 - x (the number of post-1982 pennies). Use the formula given in the introduction. Remember, do the multiplications, combine like terms, and solve for x. Because we are using rounded numbers, the answer will not be a whole number. Remember that like atoms, we are using whole pennies, so round your answers for x and 10 - x to the nearest whole number.
  3. Calculate the percent composition of the element "pennium" for both "isotopes" from your data. % of isotope = The number of each isotope present, divided by the total number (10), multiplied by 100
Conclusion: The envelope contains _____ pre-1982 pennies and _____ post-1982 pennies.

Analysis: Return the lab report and unopened envelope to your facilitator. With him, you will open the envelope to check your conclusion.