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I've had a number of people asking questions this week dealing with
conc. units<br>
and conversions between those units. Part of the problem was people
having<br>
trouble remembering what those units are. Without that knowledge
there's<br>
a lot of things you can't do. These units can be used as conversion
factors<br>
in problems. Also, if you don't know the units themselves it's
impossible to<br>
do conc. conversion problems.<br>
<br>
One other thing I noticed was people having problems with density,
sometimes<br>
not knowing exactly what it was but more often confusing density of
solution<br>
with density of solvent. You need to read the question carefully as
it will tell<br>
you whether it's the density of the solvent, solute or solution.
That can often<br>
be a hint as to what you have to do.<br>
<br>
You need to memorize the following conc. units if you haven't
already. Treat<br>
them like you did spelling words back in grade school.<br>
<br>
mass %: (mass solute)/(mass soln) * 100 <br>
or think of it as part per hundred<br>
ex. 10% sugar ==> (10 g sugar)/(100
g soln)<br>
<br>
units don't have to be "g", just same unit for
numerator and denom.<br>
<br>
mass soln = mass solute + mass solvent<br>
(needed when converting from or to molarity, as
is density of soln)<br>
<br>
ppm: like pph (mass solute)/(mass soln) * 10^6<br>
or (g solute)/(10^6 g soln)<br>
<br>
ppb: (mass solute)/(mass soln) * 10^9<br>
or (g solute)/(10^9 g soln)<br>
<br>
M: molarity (mol solute)/(L soln) denominator
is vol of soln<br>
<br>
<i>m</i>: molality (mol solute)/(kg solvent)
denominator is solvent only (not soln)<br>
<br>
X: mol fraction (mol solute)/(total mol soln)<br>
total mol soln = (mol solute +
mol solvent)<br>
<br>
mol % = X * 100<br>
<br>
mol fractions add to 1<br>
<br>
solubility: (g solute)/(100 g solvent) denominator is
solvent only (not soln)<br>
<br>
can be obtained by doing: (g solute/g
solvent) * 100 g solvent<br>
<br>
Do NOT confuse this with mass %.<br>
For mass % the denominator is mass of
SOLUTION<br>
<br>
For solubility the denominator is mass of
SOLVENT<br>
<br>
Hope this helps.<br>
<br>
Dr. Zellmer
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