Download Candium

Date:
CHEM
Name:
Activity: Average Atomic Mass
Candium
What’s the Average Atomic Mass?
Purpose
Overall. The overall purpose of this activity is to learn how to determine the average atomic
mass of an element. The average atomic mass is defined as the weighted average mass for all
of the isotopes of that element. The ‘weighted’ takes into account the abundance of each
element.
Specific. Just as most elements have several isotopes, the “element” candium comes in tw0
isotopic forms. In this activity, you will determine the average atomic mass for the element
Candium.
Background
Review – Atomic Structure
The number of protons (p+) determines the identity of the atom.
The number of electrons(e–) determines the charge of the atom.
The number of neutrons (no) determines the isotope of the atom.
And, mass number is the number of protons + the number of neutrons.
(Electrons have a mass of ~1/2000 and are not included in the mass number.)
An example is chlorine-37, with a charge of –1. It has 17p+, 18e– & 20no, and has a mass
35
Cl  .
number of 37. Its symbol is 17
Take a look below at the Key (right) for the periodic table (left).
In the key, the average atomic mass, 35.45 amu, is the weighted average mass for all of the
isotopes of chlorine. Presumably, most of the element is composed of isotopes having a mass
of 35. However, there are probably a smaller abundance of chlorine isotopes with an atomic
mass larger than 35.
In this lab, the two isotopes of Candium are:
Isotope-1:
Isotope-2:
Chemistry
Candium: Average Atomic Mass
p. 2
Procedure
1. Separate your element into two isotopes.
2. Find the total mass of each isotope group and record your calculations in the table below.
DATA:
Isotope
[A]
[B]
[C = B/A]
Number of
“Atoms”
Mass (grams)
Average Isotope Mass
Candium isotope 1
Candium isotope 2
TOTAL NUMBER OF
CANDIUM ATOMS:
3. Complete the following table.
[C]
[D = (A/total) * 100%]
[E = D/100%]
[F = C * E]
Average mass
of each isotope
Percent
abundance of
each isotope
(candy type)
Relative
abundance of
each isotope
(candy type)
Contribution of
mass of each
isotope to the
weighted
average
(candy piece)
Candium isotope 1
Candium isotope 2
TOTAL:
n/a
AVERAGE ATOMIC MASS (sum of contribution for each isotope) is in the above space

4. Find the average mass of all isotopes. ________________________
5. Class average for relative mass. ____________________________
6. Calculate the percent error*. _______________________________
% error 
Observed  Accepted
x 100
Accepted
* Your calculated value is the OBSERVED, the class averages would be the ACCEPTED
Chemistry
Candium: Average Atomic Mass
CHALLENGE
You will be given a sealed container that holds a mixture of ten M&M’s and Skittles. Your container
might hold any particular combination of the two “isotopes.” Your task is to determine the isotopic
composition of the element “Candium” without opening the container.
To illustrate how this is done, we can consider a mixture of our two isotopes of “Candium.” If your
container 9 M&M’s and one skittle, it would have a mass of 9 M&M’s PLUS Skittle. We would
mathematically represent the TOTAL MASS of your container to be:
TOTAL MASS = (# of M&M’s x mass of M&M) + (# of Skittles x mass of one Skittle)
Since we know our sample has ten “Atoms” we can say that:
 = # of M&M’s and (10 – ) = # of Skittles
Combine these two equations to solve in algebraic terms:
TOTAL MASS = ( x mass of one M&M) + ((10 – ) x mass of one Skittle)
Unknown Sample
Mass of Container w/ Candium
Mass of Container
Mass of Candium (SUBTRACT)
# M&M’s in Sample
# Skittles in Sample
% Composition of Candium:
M&M’s
(# M&M’s * Average Mass of
M&M’s/Total Mass of Sample) *
100%
%Composition of Candium:
Skittles
(# Skittles * Average Mass of
Skittles/Total Mass of Sample) *
100%
Show Calculations Below:
p. 3
Chemistry
Candium: Average Atomic Mass
p. 4
Analysis
1. Define isotope.
2. Explain the difference between percent abundance and relative abundance.
3. How does the average atomic mass differ from the relative atomic mass?
5. Determine the average atomic masses of the following elements. Show all your organized
work below.
Magnesium (symbol ____)
Mass Number
Exact Mass
Percent
Abundance
24
23.985042
78.99
25
24.985837
10.00
26
25.982593
11.01
Chemistry
Candium: Average Atomic Mass
Molybdenum (symbol ___ )
Mass Number
Exact Mass
Percent
Abundance
92
91.906808
14.84
94
93.905085
9.25
95
94.905840
15.92
96
95.904678
16.68
97
96.906020
9.55
98
97.905406
24.13
100
99.907477
9.63
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