[gakerty]

Hello all.  Chem dilution question.  Perhaps I am having a brain fart here.  I would like to calculate the amount (ul) of concentrated, 1.0x solution (lets call it S) to add up to a desired volume with a desired S concentration of 0.2x.  That can't change, the concentration must be 0.2x for all wells on my plate.  My total vol is 2000 ul for all wells, which doesn't change.

Here's where it gets tricky in my mind.  I already have a volume (800 ul) in my wells with a dilute S concentration ranging from .01x to 0.2x.  Based on the concentration of S in the well, what is the proper formula to calculate how much 1.0x S solution to add (with H2O making up the difference) to the 800 ul in the wells needed to add up to 2000 ul to yield a final S concentration of 0.2x?

Vol total = 2000ul.  This can't change.
Vol already in well = 800 ul.  All wells already have this volume in them.
Vol to add = 2000-800 = 1200.  This can't change.
[S in well] = conc of S already in well, which is one of the following: 0, 0.01x, 0.025x, 0.05x, 0.1x or 0.2x
[S final] = final conc of S = 0.2x  This must be the concentration of S for all wells in the total vol of 2000 ul.
Vol S to add =?
Vol H2O = 1200 - Vol S.

Thus the variable is:
Vol S to add = vol of 1.0x S to add to the well to yield Vol total (vol H2O making up the difference to total 2000 ul per well) with [S final], which is always 0.2x.  This will change based on the based on the [S in well].  This seems to not your typical c1v1 = c2v2 calc.=, but perhaps I am over thinking this.

Any help with a formula?  Ideally I would plug this into Excel.  Your assistance is greatly appreciated. 

[Arkcon]

Your question is a little all over the place.  Briefly, can you prepare an excel sheet based on your microwell plate:  starting with what it has, use c1vi=c2v2 to determine how much conc. solution to add, and compute the difference between the sum of those two volumes and the required total volume to compute how much water to also add.  This is not so much a formula I can give you, but a layout of an Excel spreadsheet that you have to build.