Name
Calculating the Average Atomic Mass Using a Mass Spectrogram
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Period --------------------
- Takes about one hour with skittles lab
Background:
High accuracy in determining masses for atoms of elements can be obtained by using a physical method of measurement in a device called a
mass spectrometer. The mass spectrometer has uses in many different fields of science. Geologists, biologists, petroleum chemists, and
many other research workers use the mass spectrometer as an analytical tool. Its development was based on the design of the early tubes 1.1.
Thomson used to find the charge/mass ratio of the electron.
Using a mass spectrometer, we can determine the relative amounts and masses ofthe nuclei for all isotopes of an element. The element
sample, which is in gaseous form, enters a chamber where it is charged by a beam of electrons. These charged particles are then propelled
by electric and magnetic fields. As in Thomson's cathode ray tube, the fields bend the path of the charged particles. The paths of the heavier
particles are bent slightly as they pass through the fields. The paths of the lighter particles bend sharper. Thus, the paths of the particles are
separated by relative mass and then detected and recorded electronically. One drawback of the instrument is that the charging chamber, filed
tube, and detection device must all be in a vacuum. The vacuum containing these devices must be equal to about one hundred-millionth
(1/1 00 000 000) of normal atmospheric pressure. A mass spectroscopy readout can be seen in figure 1 below.
Because the strength of the magnetic and electric fields as well as the speeds and paths of the particles are known, the mass of the particles
can be calculated. Once the masses of the isotopes and their relative amounts have been found, the average atomic mass can be calculated.
This average atomic mass is called the atomic mass of an element.
Not all isotopes of an element are present in the same amount. Thus, in finding average atomic masses we use what is called a weighted
average.
Figure 1 is a mass spectrogram. It is the printout of the results of a mass spectrometer's analysis of a naturally occurring element. The
spectrometer detected seven different isotopes which are identified by number. The height ofthe graph of a specific isotope reflects the
relative amount of that isotope. So, for instance, if the spike of one isotope is twice the height of the spike from another isotope, then twice
as much of the first isotope existed in the sample as the second isotope.
Purpose:
You will be measuring the heights of the graph in millimeters and comparing them to each other. Then you will calculate, based on your
measurements, the average atomic mass of the naturally-occurring element. Identify it. Then determine the percent error you had in
obtaining the amu. You will then determine the amu of a fictional element using candy or cereal. You will then create a mass spectrogram of
this element.
Name
_
Pe riod
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Data/Analysis:
Mass Spectrometry
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Pre-lab
Procedure (Part I):
1. Carefully measure in millimeters the height of the spike
corresponding to each isotope. Record it in the data table
below. Measure form the black x-axis (light dark
horizontal line) to the top of the spike. Remember to
measure to the nearest tenth of a millimeter.
Add all the heights together and record the total at the
bottom of the column.
3. Calculate the fraction abundance of each isotope by
dividing the height of its spike by the total of all the
heights. (Height of spike/total height).
Then total this column.
This total should be
between 0.98 and1.02.
4. Calculate the percent abundance of each isotope by
taking the fraction abundance and multiplying it by 100.
( fraction abundance x 100) Then total.
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Figure 1
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204
2.
5.
6.
7.
a. This total should be between 98% and 102%.
Multiply each isotope's fraction abundance by its
mass number and record the result in the last column of
the data table.
Then add these together for the total mass (amu). Use
this number to determine what the element is on the
periodic table (look at the mass number)
Check your Element and AMU with me then calculate
the percent error.
Show your work
% Error = I Expected - Observedl
Expected
x 100
196
J~ ..\-I"-"
198
.---
200
202.
mle
Mass
# of
Isotope
Height
of each
peak in
mm
Fraction
abundance
Percent
abundance
A.
204
Fraction abundance
x mass #
196
198
199
200
201
202
204
Total
AMU (5 sig fig's)
Element Name
% error
I
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p. --.r
Iflfl'l.."l •••----------
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GFinding
the AMU of Candium
The element is Candium (the bag of skittles).
identical but have slight differences
Inside each element is atoms. The atoms are chemically
in their Masses. Use their fraction of abundance to calculate their
AMU.
Color
(isotope)
1
2
Mass
of
Your
#
isotope
3
Another
person'
s
4
Anothe
r
person'
#
s
#
Red
45
Yellow
44
green
42
orange
40
purple
37
Total
XXXX
XXXX
XXXX
XXXX
5
Average the 3 bags
Columns
1+2+3
3
Round to 4 digits or don't
round at all
Add up this column
6
7
Fraction of
Fraction
abundance
Mass
Each of the
Column 5
numbers divided
by the column
total
(round to 5 digits
or don't round)
This should
adduptol
lx6
(round to 5
digits or don't
round)
Total AMU
(5 digits)
Your Total AMU is your Observed.
Your Expected is 42.513
The element would fit inbetween
what two elements on the periodic table?
Show your work and calculate your percent error then circle your answer.
Expected - Observed
Expected
(absolute value)
X 100 =
and
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