Name …………………………………………………………….
AP Chemistry
Mass Spectrometry, Percent Abundance & Atomic Mass
Mass spectrometry is a technique used in chemistry to determine average atomic mass values for the elements. It can also be
used to identify unknown compounds, such as in forensics and archeology.
How a Mass Spectrometer Works:
• A gaseous sample is hit with electrons, which produces
positive ions
• These positive ions are sent through a magnetic field, which
exerts a force on the charged ions
• The ions are deflected by the force of the magnetic field and
hit a film on the other side of the magnetic field – this is how
they are detected
• The lighter (less massive) particles are affected by the force
of the magnetic field more than the heavier particles, and so
will be deflected more from the straight-line path.
• By comparing the amount or degree of deflection for the
particles in a sample of a given element to the standard
deflection angles for a sample of carbon-12, atomic masses*
can be determined. *These are relative atomic masses –
relative to the standard (carbon-12).
• From the number of hits at each deflection point, the
relative abundances of the isotopes in the sample can be
determined.
Mass Spectrometer
Mass Spectrometer Reading for Neon
How to Read a Mass Spectrogram:
• A peak indicates that particles with that mass number were
detected
• The taller the peak, the more particles there were in the sample
with that particular mass
• This spectrogram for neon shows a very tall peak at mass = 20 and
a short peak at mass = 22. This means that there were more neon20 atoms than neon-22 atoms. The neon-20 isotope contributes
more to the average atomic mass for neon because there are more
atoms of neon-20 than neon-22. Looking up the average atomic
mass for neon on the Ref. Tables, we find that it is 20.18 amu,
which makes sense because there are many more neon-20 atoms
than neon-22 atoms, making the average atomic mass closer to 20
For example:
than to 22.
• You may be asked to pick out a correct spectrogram for an
Which mass spectrometer reading represents naturally occurring
magnesium?
element,
given its atomic mass, on the AP Exam.
(A)
(B)
(C)
(D)
Steps for Calculating Average Atomic Mass (when given the masses and percent abundances):
1. Convert the percentages into decimals by dividing by 100 (move decimal over two spaces to the left)
2. Multiply the decimal of each isotope by its respective mass.
3. Add the numbers from step two together. (You can punch the numbers into your calculator as indicated in the
equation below.
Equation: Average atomic mass = mass of isotope 1*decimal abundance + mass of isotope 2* decimal abundance +…
Example1: Calculate the average atomic mass for silicon using the data in the table below:
Example 2: Calculate the average atomic mass for iron using the data in the table below:
Steps for Calculating Percent Abundance (When given molar masses OR mass in amu for each isotope):
1. Because decimal abundances will always add up to 1, assign one isotope to have an abundance of “x” and the other
isotope to have an abundance of “1-x.”
2. Look up the average atomic mass of the atom on the periodic table if it’s not given in the problem.
3. Set up your problem so it looks like the setup below:
(Mass of Isotope A) * x + (Mass of Isotope B) * (1-x) = Average Atomic Mass
4. Solve for “x.” Multiply this number by 100 to turn it into a percentage. This is the percent abundance of isotope A.
5. The percent abundance of isotope B is 100% minus the percent abundance of isotope A.
Example 3: Gallium consists of two naturally occurring isotopes, Ga-69 and Ga-71, with respective masses of 68.926 and
70.925 amu. Calculate the abundance of each isotope.
Example 4: Oxygen comes in two stable isotopes, Oxygen-16 and Oxygen-18. The molar mass of Oxygen-16 is 15.98
g/mol. The molar mass of Oxygen-18 is 17.99 g/mol. Determine the percent abundance of each isotope.
Steps for Calculating Numbers of Atoms in a Given Sample (When given mass of sample & % abundance):
1. Convert grams to moles, by dividing the mass of the sample by the AVERAGE atomic mass. (1 mol of naturally
occurring element = GFM = average atomic mass)
2. Multiply by the decimal abundance of the isotope you are solving for. (You can think of this as a mole ratio – for every
one mole of naturally occurring element, there are x moles of the isotope present.)
3. Multiply by Avogadro’s number to get to atoms.
Example 5: Determine the number of Ga-69 atoms in a 20. gram sample of naturally occurring gallium (using your answer to
Example 3).
Example 6: Determine the number of oxygen-18 atoms in a 12 gram sample of naturally occurring oxygen (using your
answer to Example 4).