calculations

Leman Leman at Leman.org
Thu Jun 28 14:13:30 EST 2001


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.
The most common one is making final solution from a stock. Well, if
your stock is 5X and your final volume is 1 L, take 1000 mL / 5 = 200
mL of stock and bring the volume up to 1 L with whatever else must be
there.
The reverse calculations are as easy: if you use 2 mL of 10% SDS stock
to make 50 mL of your solution, in the end it will have 0.4% SDS. Why?
Because 2 mL out of 50 mL means 25-fold dilution and 10% / 25 = 0.4%

Of course, one can forget this simple logic and do V1*C1=V2*C2
calculations, but I think logic is better than algebra in this case,
since it doesn't let you make a silly mistake like dividing by
something when you have to multiply (which might accidently happen if
you mechanically rearrange equations on paper without ever thinking
what it all means in a beaker).

If something has molecular weight of 45 and I need to make 30 mL of 20
mM solution, one can dig up an equation and plug in the numbers (and
often be off by a factor of 10 or 100 or even 1000 because of a simple
mistake) or do it the old-fashioned way: using your own head and a
piece of paper. MW of 45 means 1 M solution will have 45 g per 1 L, or
45 mg per 1 mL (simple: divide mass and volume by 1000). Then, 1 M =
45 mg / 1 mL or 450 mg / 10 mL. 10 mM is 100 times less than 1 M, so
10 mM = 4.5 mg / 10 mL (you can multiply or divide by 10 as many times
as you need with your eyes closed) and 20 mM = 9.0 mg / 10 mL
(multiplying by 2 is not too hard either). Alright, we just "made" 10
mL of our solution! We'll need 3 times as much for 30 mL, therefore
we'll weigh 9 * 3 = 27 mg, dissolve it in 25 mL of the solvent, bring
the volume up to 30 mL and we're done. Not a single equation to look
up, not a single reason to use a calculator! Only simple logic and
calculations you can do in your head or on a paper towel.

That pretty much covers most of it (that and the fact that 1% = 10 g/L
or 10 mg/mL :-))

Leman




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