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Isotopes and Atomic Mass

1. Determine the average weight of each isotope of the fictitious element vegium.

2. Determine the relative abundance of isotopes of vegium.

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3. Calculate from experimental data the atomic mass of vegium.

A sample of vegium on a plastic cupsmall-scale balance
1. Weigh all the beans, all the peas, and all the corn.

2. count all the beans, all the peas, and all the corn.

3. Divide all the mass of each by the isotope (beans, peas, and corn) by the number of each isotope to get
the average mass of each isotope.

4. Divide the number of each isotope by the total number of particles, and multiply by 100 to get the
percent abundance of each isotope.

5. Divide the percent abundance from step 4 by 100 to get the relative abundance of each isotope.

6. Multiply the relative abundance from step 5 by the average mass of each isotope to get the relative
weight of each isotope.

7. Add the relative weights to get the average mass of all particles in vegium, the “atomic mass.” Note:
When you weigh the various types of vegetables, you may encounter some problems. For example, the
sample of beans might be too large to weigh on your balance. You might solve this problem by making
more weights or by using a larger counterweight on your balance. This approach increases your balances
capacity. Keep in mind that it also results in a heavier beam, which reduces the sensitivity of your balance.

Alternatively, you might weigh a portion of your vegetables, say half, and then multiply your result by two
(or a fifth and multiply by five). The beans are various in sizes, so if you weigh just one bean, and
multiply by the number of beans to get the total weight of beans, a significant error might result. Weigh a
large enough sample so you get a good estimation of the average weight of a bean.

BeansPeasCorn Total
Mass of each isotope19.2g15.2g36.1g70.5g
Number of each isotope68186216470
Average mass of each.2823529g.0817204g.1671296g.15g
Percent of each14.468%39.574%45.957%99.999%
Relative Abundance.14468.39574.45957.99999
Relative Weight.0408508g.032407g.0768078g.1499985g
We followed the directions listed in procedures. To get the Relative abundance, we divided by 100. About
the same as moving the decimal to the left 2 places. To ensure that the answers were right, the percent of
each is supposed to be as close to 100% as possible. We came very close.

In determining we found all the averages of the weights of each isotope. In calculating the
experimental data the atomic mass of vegium, we answered that question successfully. And in the second
determining, we determined the relative abundance of isotopes successfully.

1. While counting the beans, there were more than 450 total pieces, so while counting we could have
miscounted because the half-peas looked a lot like corn.

2. While weighing, we had to tare the beaker, This can sometimes prove to be inaccurate.

3. The percent of each row, the total is 99.999%, to be 100% accurate, the number has to be 100%. This
proves that somewhere down the line, we didnt calculate right. The probable cause for this is that we
didnt list all numbers. The real numbers were over 10 characters long, we used SigFigs.


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