conc units

[robert zellmer]

I've had a number of people asking questions this week dealing with 
conc. units
and conversions between those units.  Part of the problem was people having
trouble remembering what those units are.  Without that knowledge there's
a lot of things you can't do.  These units can be used as conversion factors
in problems.  Also, if you don't know the units themselves it's 
impossible to
do conc. conversion problems.

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 have to do.

You need to memorize the following conc. units if you haven't already.  
Treat
them like you did spelling words back in grade school.

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

                   units don't have to be "g", 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)

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

                                     mol fractions add to 1

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

Hope this helps.

Dr. Zellmer
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