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Isotopes Lab

PURPOSE:

This exercise will demonstrate the relative abundance of isotopes and explain how the weighted atomic mass of the elements is determined. Pennies from 1982 are similar to a mixture of isotopes in that they all look the same, but their masses tend to group around two values rather than one. This difference is due to a change in the minting process from solid copper to copper with a zinc core. Therefore, holding two pennies, one that is solid copper and one that is copper with a zinc core is similar to holding isotopes of an element in your hand.

MATERIALS:

balance; 20 pre-1982 pennies; 20 post-1982 pennies; 20 1982 pennies.

PROCEDURE:

  1. Work in pairs and obtain 20 pre-1982 pennies. Record their average weight to the nearest 1/100th of a gram. This represents the lighter isotope we will call "X".
  1. Now obtain 20 post-1982 pennies and record their average weight to the nearest 1/100th of a gram. This represents the heavier isotope that we will call "Y" (example 3.24g).
  1. Next obtain 20 1982 pennies and record their average weight to the nearest 1/100th of a gram. Here we have mixed isotopes with an average weight of (3.09g)
  1. Determine the relative abundance of each isotope using the following formula:

Use the following formula to determine the percent of the mixed group that are pre-1982 type isotopes and post-1982 type isotopes.

avg. wt. of 1982 coins = [avg. wt. of "Y" + avg. wt. of "X" / 2 ]

For the avg. wt. "X" we can substitute (1 - Y) because "X" can be determined by subtracting "Y" from 100%.

Example: 3.09 = [3.24 + (1 - 3.24 X) / 2 ]

  • 3.09 = [3.24 + (-2.24 X) / 2 ] Then subtract 3.24 from both sides of the equation
  • -0.15 = [-2.24 X / 2]
  • Divide the "X" value by 2 and then solve for "X" by dividing both sides by the number in front of the "X" eg. (-1.12)
  • 0.134 = "X" change to percent by moving the decimal = 13.4%
  • Therefore, "X" = 13.4% so "Y" must equal (100% - "X") or 86.6%.
  • CONCLUSION:

    1. Why is the average weight used instead of the weight of one coin?

    1. How can you tell pre-1982 from post-1982 coins without looking at the date or weighing them?

    1. Do the following problems and find the atomic mass using significant figures.

    75.53% 35.0Cl and 24.47% 37.0Cl and also 19.6% 10.0B and 80.4% 11.0B