Name _ Period --------------------

Calculating the Average Atomic Mass Using a Mass Spectrogram - 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 amass spectrometer. The mass spectrometer has uses in many different fields of science. Geologists, biologists, petroleum chemists, andmany 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 elementsample, which is in gaseous form, enters a chamber where it is charged by a beam of electrons. These charged particles are then propelledby electric and magnetic fields. As in Thomson's cathode ray tube, the fields bend the path of the charged particles. The paths of the heavierparticles are bent slightly as they pass through the fields. The paths of the lighter particles bend sharper. Thus, the paths of the particles areseparated by relative mass and then detected and recorded electronically. One drawback of the instrument is that the charging chamber, filedtube, and detection device must all be in a vacuum. The vacuum containing these devices must be equal to about one hundred-millionth(1/100 000 000) 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 particlescan 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 weightedaverage.

Figure 1 is a mass spectrogram. It is the printout of the results of a mass spectrometer's analysis of a naturally occurring element. Thespectrometer detected seven different isotopes which are identified by number. The height ofthe graph of a specific isotope reflects therelative amount of that isotope. So, for instance, if the spike of one isotope is twice the height of the spike from another isotope, then twiceas 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 yourmeasurements, the average atomic mass of the naturally-occurring element. Identify it. Then determine the percent error you had inobtaining the amu. You will then determine the amu of a fictional element using candy or cereal. You will then create a mass spectrogram ofthis element.

Name _ Pe riod _

Mass Spectrometry

Pre-lab Procedure (Part I):1. Carefully measure in millimeters the height of the spike

corresponding to each isotope. Record it in the data tablebelow. Measure form the black x-axis (light darkhorizontal line) to the top of the spike. Remember tomeasure to the nearest tenth of a millimeter.

2. Add all the heights together and record the total at thebottom of the column.

3. Calculate the fraction abundance of each isotope bydividing the height of its spike by the total of all theheights. (Height of spike/total height).

Then total this column.This total should bebetween 0.98 and1.02.

4. Calculate the percent abundance of each isotope bytaking the fraction abundance and multiplying it by 100.( fraction abundance x 100) Then total.

a. This total should be between 98% and 102%.5. Multiply each isotope's fraction abundance by its

mass number and record the result in the last column ofthe data table.

6. Then add these together for the total mass (amu). Usethis number to determine what the element is on theperiodic table (look at the mass number)

7. Check your Element and AMU with me then calculatethe percent error.

Show your work% Error = I Expected - Observedl x 100

Expected

Data/Analysis:·202

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0> -I

Figure 1·199

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204A.J~ ..\-I"-".--- 200 202.

mle 204196 198

Mass Height Fraction Percent Fraction abundance# of of each abundance abundance x mass #Isotope peak in

mm196198

199200

201

202204

TotalAMU (5 sig fig's)

Element Name % error I

)J Iflfl'l.."l •••---------- p. ~$j --.rGFinding the AMU ofCandium

The element is Candium (the bag of skittles). Inside each element is atoms. The atoms are chemically

identical but have slight differences in their Masses. Use their fraction of abundance to calculate their

AMU.

Color 1 2 3 4 5 6 7(isotope) Mass Your Another Anothe Average the 3 bags Fraction Fraction of

of # person' r Columns abundance Massisotope s person' 1+2+3 Eachof the lx6

# 3 Column 5 (round to 5s Round to 4 digits or don't numbers divided# round at all by the column digits or don't

total round)(round to 5 digitsor don't round)

Red 45

Yellow 44

green 42

orange 40

purple 37

Total XXXX XXXX XXXX XXXX Add up this column This should Total AMUadduptol (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? and _

Show your work and calculate your percent error then circle your answer.

Expected - Observed (absolute value) X 100 =Expected