conc units

[Zellmer, Robert]

I always get questions from students dealing with conc. units and
conversions between those units after the quiz.  Part of the problem is
people having trouble remembering what those units are.  Without this
knowledge you'll have trouble doing certain things.  These units can be
used as conversion factors in problems.  Also, if you don't know the units
themselves it's impossible to do any of these problems.  These units are
given at the end chapter 13 in the "Key Equations" list.

One other thing I noticed was people having problems with density, sometimes
not knowing exactly what it was but more often confusing density of solution
with density of solvent.  You need to read the question carefully as it will tell
you whether it's the density of the solvent, solute or solution.  That can often
be a hint as to what you must do.  People quite often miss the conc.
conversion problems.  Since I can't see the work for a quiz or exam I'm not
sure why but often it's because people don't know the units themselves.

You need to memorize the following conc. units if you haven't already.  Treat
this like you did spelling words back in grade school.  We do provide some
equations on exams but not all and not usually these.

mass %:        (mass solute)/(mass soln) * 100
                        or think of it as part per hundred (pph)
                        ex.    10% sugar  ==>  (10 g sugar)/(100 g soln) by mass

                  units don't have to be "grams", just same unit for numerator and denom.

                    mass soln = mass solute + mass solvent
                    (needed when converting from or to molarity, as is density of soln)

ppm:        like pph        (mass solute)/(mass soln) * 10^6
                                    or (g solute)/(10^6 g soln)

Also, mg solute/L soln is considered a mass/vol ppm.  For
dilute solns where the density of soln is essentially the same
as for pure water (1 g/ml), this is essentially the same as
mg solute/kg soln

ppb:                            (mass solute)/(mass soln) * 10^9
                                    or (g solute)/(10^9 g soln)

M:        molarity        (mol solute)/(L soln)        denominator is vol of soln

m:        molality        (mol solute)/(kg solvent)    denominator is solvent only (not soln)

X:        mol fraction        (mol solute)/(total mol soln)
                                          total mol soln = (mol solute + mol solvent)

                                    mol % = X * 100

                                    mole fractions add to 1, mole percents add to 100

solubility:        (g solute)/(100 g solvent)    denominator is solvent only (not soln)

                        can be obtained by doing:        (g solute/g solvent) * 100 g solvent

                        Do NOT confuse this with mass %.

                        For mass % the denominator is mass of SOLUTION

                        For solubility, the denominator is mass of SOLVENT

In terms of converting between the units I suggest you try doing them the way I
did in class.  While there are other ways of doing them, I've provided a single
road map on how to do all of them.  It's easier to learn one way to approach all
these problems than several different ways based on what your given and what
you're after.

Hope this helps.

Dr. Zellmer
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