Download Quantities in Chemical Reactions

Isotopic Abundance
SCH 3U
Atomic Mass
• The mass of an atom (protons, neutrons,
electrons)
Relative Atomic Mass:
• An element’s atomic mass relative to the
mass of a carbon-12 atom
Atomic Mass Units (amu or u)
• A unit of mass defined as 1/12 the mass of
one C-12 atom OR 1 proton or neutron
• E.g. C-12 has a mass of 12 X 1 u = 12 u
E.g. The mass of a sodium atom is 1.92 times
that of C-12. Calculate the atomic mass of
sodium in atomic mass units.
mc-12 = 12u
mNa = 1.92 X 12 u = 23.04 u
Compare this to the average atomic mass.
22.99 u
What accounts for the difference?
It’s an average of an element’s isotopes
Isotopic Abundance
• For most elements, the isotopic composition
remains constant
• E.g. any sample of naturally occurring carbon
has the same percentage of C-12 and C-13
• Because this is constant, we can use an
average value for the atomic mass of carbon
• The percentage of an isotope in a sample is
called its isotopic abundance
E.g. Given
Isotopic Abundance of C-12 = 98.89%
Isotopic Abundance of C-13 = 1.110%
Calculate the average atomic mass of carbon.
mavg = [mC-12 X %C-12] + [mC-13 X %C-13]
= [12 u X 0.9889] + [13 u X 0.0110]
= 11.8668 u + 0.1443 u
= 12.01 u
E.g. Chlorine has 2 isotopes,
Isotopic Abundance of Cl-35 = 75.53%
Isotopic Abundance of Cl-37 = 24.47%
Calculate the average atomic mass of chlorine.
mavg = [mCl-35 X %Cl-35] + [mCl-37 X %Cl-37]
= [35 u X 0.7553] + [37 u X 0.2447]
= 26.4355 u + 9.0539 u
= 35.49 u