Warmup
Turn in the SISS lab!
Next class, bring in colored pencils if you have them.
You can chew gum today for Sugar Rush. At the end of class, you
can weigh your gum samples or leave them to dry, whatever you
need. Store anything in the back cabinet.
Atom A has 37 protons and a mass of 70 amu.
Atom B has 33 protons and a mass of 66 amu.
Atom C has 33 protons and a mass of 70 amu.
1. Are these atoms all of the same element? Explain.
2. Calculate the number of neutrons in each atom
3. Silently read the introduction to “Average Atomic
Mass and Percent Abundance” homework and
locate your calculator.
Average Atomic Mass
1. The atomic mass of copper 63, is 62.93 amu, and has
an abundance of 69.09%. Approximately 30.91% of
copper atoms exist as copper 65, having an atomic mass
of 64.9278 amu. The atomic mass shown on the periodic
table for copper is 63.55 amu. How did scientists arrive at
this value?
average atomic
mass for an
element
% abundance
of isotope 1 (in
decimal form)
=
average atomic
mass for Cu
=
average atomic
mass for Cu
a.a.mCu
0.6909
atomic mass
of isotope 1
x
x
62.93
43.478337
=
=
63.55 amu
% abundance of
isotope 2 (in
decimal form)
+
+
+
0.3091
atomic mass
of isotope 2
x
x
20.06918298
64.9278
2. Naturally occurring sulfur consists of four isotopes, S-32 (95.0%),
S-33 (0.76%), S-34 (4.22%), and S-36 (0.014%). Using these data,
calculate the atomic weight of naturally occurring sulfur. The
masses of the isotopes are:
S-32: 31.97amu
S-33: 32.97amu
S-34: 33.97amu
S-36: 35.97amu
%
abund.
S-32
0.950
x
x
31.97
amu
31.97
amu
30.3715
+
+
%
abund.
S-33
x
0.0076
+
32.97
amu
x
+
%
abund.
S-34
32.97
+ 0.0422
amu
0.250572
+
33.97
amu
x
x
1.433534
+
33.97
amu
+
32.06 amu (very close to the PT value)
+
%
abund.
S-36
x
0.00014
0.0050358
35.97
amu
35.97
amu
x
=
=
=
3. Rubidium has only 2 known stable isotopes.
Calculate the % abundance of each isotope
85Rb = 84.9118 amu
87Rb = 86.9092 amu
% abundance
of isotope 1 (in
decimal form)
x
atomic mass
of isotope 1
x
x
84.9118
amu
% abundance of
isotope 2 (in
decimal form)
+
+
y
atomic mass
of isotope 2
x
x
86.9092
amu
average atomic
mass of Rb
=
=
85.47 amu
y=1-x
x
x
84.9118
amu
+
1-x
x
86.9092
amu
=
85.47 amu
84.9118x + 1(86.9092) – x(86.9092) = 85.47
84.9118x + 86.9092 – 86.9092x = 85.47
x = 0.721, y = 0.279
-1.9974 x = -1.4392
-1.9974
-1.9974