calculations

Frank O. Fackelmayer Frank at Fackelmayer.de
Fri Jun 29 04:13:00 EST 2001



Leman wrote:
> 
> On 25 Jun 2001 11:54:22 GMT, khaq at uoguelph.ca (KH) wrote:
> 
> >Can anyone suggest a good site or book where I can learn some basic
> >chemical calculations? I do it is absurd to be working in a lab and yet
> >having problems with simple chemical calculations! Stuff like making
> >stock solutions, calculation # of moles, volumetric equations etc.
> 
> I would imagine any Chem101 texbook will do.
> 
> There are really 3-4 types of calculations one uses in the lab every
> day.

- snip descriptions -


agreed. That´s the fastest way to do the basic calculations you need
every day.

One more thing that comes to my mind is making a buffer of a certain
molarity from a liquid like acetic acid, where you "don´t know" the
molarity of the stock. Of course it is trivial, but I have seen so many
people with this problems that I think it is ok to post the solution here...

Read the label to find the values for density and FW. Divide density by
FW to get the molarity. Example: glacial acetic acid has a density of
1050g/l and a FW of 60.05g/mole; the molarity is 1050/60.05 = 17.5M.
Then simply use the "dilution factor" method Leman has described above
to determine how much of the stock you need to make a buffer of the
wanted molarity. Example: you want 250ml of 50mM acetic acid. Dilution
factor is 17500/50=350; thus, you take a threehundredfiftieth of the
final volume, or 250/350= 0.714ml of the stock. 
Of course you can calculate it directly with the V1*C1=V2*C2 formula,
but I agree with Leman that you can more easily understand it the
"dilution" way. 

Frank

(apologies to all those who find it too trivial to mention here...)




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