# [alt-photo] Re: Stoichiometry for the nonscientist

etienne garbaux photographeur at nerdshack.com
Mon Aug 2 05:27:12 GMT 2010

```David wrote:

>OK - What if I needed to mix a formula like: 180 gms potassium oxalate
>K2C204.H2O mixed with water to make 1 liter.  I only have potassium
>carbonate and oxalic acid on hand.
>
>I know that I can make this formula of potassium oxalate by mixing
>potassium carbonate and oxalic acid; PC is K2CO3 and OC is C2H204.
>Now how would I know how much of each of these to make up my original
>formula?

As Eric already said, you have to figure out what happens to
everything you put into the reaction.  You also need to know the
thermodynamics of the reaction you are contemplating -- whether it is
exothermic or endothermic, and by how much.  Then, it is a matter of
figuring how much of each reactant you need.  That is where mols come
in (which, as I recall, is what started this thread).  "180 grams"
doesn't tell you how many molecules of potassium oxalate you hope to
end up with and, consequently, how many molecules of each reactant
you need.  No getting around it, you need to figure this out using
the molecular weights of the compounds involved.  You can do this
every time you have a chemical weight (like "180 grams of potassium
oxalate," or you can do it beforehand by making up solutions that
have a known number of molecules per milliliter and then just
measuring a volume of each solution.

That's what the concept of "molar concentration" is.  Instead of
counting molecules, we find it easier to count by bunches of
molecules -- 6.022 x 10^23 molecules, to be precise.  That is handy
because of one Sig. Amedeo Avogadro and one M. Jean Perrin, who
brought us the concept of "gram molecular weight" ("GMW" or
mol).  GMW is simply the realization that 6.022 x 10^23 atoms or
molecules weigh, in grams, the atomic or molecular weight of the atom
or compound.  So, 58.443 grams of table salt (NaCL), which has a
molecular weight of 58.443, will have 6.022 x 10^23 molecules (give
or take one or two).  If you then dissolve this salt in water and add
water to make one liter, you have a "one molar" solution -- it
contains one GMW of salt in every liter.  If you dissolve 2 x 58.443
= 116.89 g and make up one liter, you have a "two molar" solution,
and so forth (limited, of course, by the solubility of the compound
in question).

By inspection, you can see that 1 mol of potassium carbonate reacts
with 1 mol of oxalic acid to make 1 mol of potassium oxalate, with
one mol of H2 and one mol of CO3 left over.  The H2 and one oxygen
atom react to make one mol of water (H2O), leaving only carbon
dioxide (CO2) in excess.  So, we expect this reaction to evolve one
mol of CO2 for every mol of potassium carbonate and oxalic acid we react.

The molecular weight of anhydrous potassium oxalate is 166; the
monohydrate is 184.  Thus, the solution you postulated is very close
to 1 molar (0.978 molar).  If you react 489 ml of 2 molar potassium
carbonate 489 ml of 2 molar oxalic acid, then add water to make 1
liter, you would have what you want.

Note that if you started with potassium hydroxide (KOH) instead of
potassium carbonate, you would require two mols for each mol of
oxalic acid and the only byproduct would be water.

Best regards,

etienne

```