Making a solution - (Sep/07/2006 )
I am not used to making solutions 
so I have a rather simple question.
How do I make a 200 ml solution of 4% of PEG? What does the percentage correspond to? I assume it is the weight? Is it then 8 grams PEG in 200 ml H2O?
Thanks in advance,
I think you are correct 
that will be a w/v solution (for Weight per Volume; as opposed to v/v or w/w)
please note that the total volume will be 200 mL; it is good to add the 8gm to a smaller volume (like 180mL), mix into solution, and then top off to 200mL using a graduated cylinder
Aimikins is correct.
% solution (w/v) is g/100ml. Therefore, 4% PEG is 8g/200ml as you have stated.
It will depend on the mean molecular weight of your PEG.
If you have something like PEG 400 it will come as a liquid (so you'll be making v/v), if it's something like PEG 8000 which comes as large solid flakes then you'll be making w/v.
If you have something like PEG 400 it will come as a liquid (so you'll be making v/v), if it's something like PEG 8000 which comes as large solid flakes then you'll be making w/v.
even if it is a liquid you can (and maybe should, to be consistent) do w/v. just weigh the liquid or calculate by density. we weigh glycerol for some solutions so that we don't have to wait for it to finish dripping out of the pipet or cylinder.
It will depend on the mean molecular weight of your PEG.
If you have something like PEG 400 it will come as a liquid (so you'll be making v/v), if it's something like PEG 8000 which comes as large solid flakes then you'll be making w/v.
even if it is a liquid you can (and maybe should, to be consistent) do w/v. just weigh the liquid or calculate by density. we weigh glycerol for some solutions so that we don't have to wait for it to finish dripping out of the pipet or cylinder.
Weighing liquid PEG without knowing its density isn't that advisable, you'll be weighing solvent as well as solute. Knowing its density can be a problem - I've seen many a jug of PEG that only tells you the mean molecular weight.
You're absolutely right about glycerol. It's near to impossibly if you were foolish enough to try measure it by volume - it's far too viscous for that. A quick calculation with density should tell you how much you need to weigh to get the correct volume.
However, if Edwige is using PEG 200 then it's not a problem just to measure its volume and then rinse the measuring vessel a few times with minimum amounts of solvent (retaining the washes, of course).
if you pipet (or measure by cylinder) for v/v you would still have the problem of solvent, if there is any. peg-400 is not in solvent (unless you buy it diluted), it is liquid. you can weigh, pipet or pour it. if you don't pour it directly into your final vessel then you will have to rinse to get it all out. it is just as easy to weigh it into the final vessel prior to any other addition.