My previous post addressed the calculation of
molecular masses from the atomic masses in the
periodic table. This post addresses a different
issue raised by Bob Cohen in his original post...
Are the isotopic abundances for a substance the
same in the solid, liquid, gas phase?
I believe the answer is yes and no. And, when the
answer is no, how much of a difference we see
depends on the size and type of molecule.
In the case of water, a small molecule, where
hydrogen bonding is important, the intermolecular
attractions between 1H----16O and 2H----16O
are different. This is one example of the "isotope
effect." The hydrogen bonding between 1H----16O
and 1H----18O is also different, but not as much.
As the atom gets larger, the isotope effect gets
smaller.
Anyway, since the isotope effect exists, the vapor
pressure of H2O is different than the vapor pressure
of D2O. Therefore the isotopic abundance in the
vapor is slightly different than the isotopic abundance
in the liquid IF we have liquid/vapor equilibrium, or
if we are in the process of evaporating or condensing.
This means the history of a sample is important. Suppose
we take natural hydrogen and combust it with natural
oxygen and we condense ALL the water as liquid. This will have
the natural abundance of H and O isotopes. If we freeze
it all, then the solid has the natural abundance. If we
boil it all, then the vapor has the natural abundance.
However, if we take the liquid and let SOME of it evaporate
then neither the vapor nor the liquid will have the natural
abundance.
I would think the difference between CO2 gas and CO2
solid would be very slight as far as isotopic abundances
goes. The intermolecular forces in this case are different
in nature (not hydrogen bonding) and the atoms involved
are larger and therefore show a smaller isotope effect.
These effects are small and you need mass spectrometers
to see the difference. You won't see the difference by
weighing with an analytical balance. And, I picked water
because it shows a greater isotope effect than
most things.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu