stas - Le Moyne

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Stas atomic weights
Solutions
1) Since silver chloride contains equal numbers of silver and chlorine atoms, the mass
ratio of silver to chlorine in a sample of AgCl is the same as the mass ratio of one atom of Ag to
one atom of Cl or a mole of Ag atoms to a mole of Cl atoms. Set up a proportion:
100.000
x
,

32.8845 35.500
where the ratio on the left is the Ag:Cl mass ratio in the analytical sample and the ratio on the
right is the ratio of an atom of Ag to an atom of Cl. The mass of an atom of silver is what we are
looking for. Solving for x yields:
100.000
atomic weight of Ag = x = 35.500 
= 107.95 .
32.8845
2) If silver sulfide contains equal numbers of silver and sulfur atoms, the mass ratio of
silver to sulfur in a sample of silver sulfide is the same as the mass ratio of one atom of Ag to
one atom of S or a mole of Ag atoms to a mole of S atoms. Set up a proportion as before:
100.000 107.95

,
14.852
y
where the ratio on the left is the Ag:S mass ratio in the analytical sample and the ratio on the
right is the ratio of an atom of Ag to an atom of S. The mass of an atom of sulfur is what we are
looking for. Solving for y yields:
14.852
atomic weight of S = y = 107.95 
= 16.033 .
100.000
But the formula for silver sulfide is really Ag2S, so the mass ratio in the analytical sample
is equal to the ratio of the mass of two atoms or moles of silver to that of one of sulfur. So the
correct proportion is:
100.000 2 107.95

,
14.852
y
and the correct atomic weight is:
14.852
atomic weight of S = y = 2 107.95 
= 32.065 .
100.000
3) Mass relationships exist not just within compounds (that is between components of a
compound) but also between compounds in a reaction. In the precipitation of AgCl by KCl and a
silver salt, the mass of KCl represents the same number of moles as the mass of silver. So we
can set up proportions as before:
100.000 107.95

,
69.103
z
where z is the molecular weight of KCl.
69.103
z = 107.95 
= 74.597 .
100.000
The atomic weight of potassium is this molecular weight minus the atomic weight of chlorine:
atomic weight of K = 74.597 – 35.500 = 39.097 .
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