Calculation help - (Nov/11/2008 )

So I need to deactivate some DNaseI by adding EDTA and EGTA to a sample so that the final concentration in the sample is 100mM EDTA and 100mM EGTA. How do I calculate this with 500 mM stocks?

With C1V1=C2V2 I get

(500mM)(x)=(100mM)(10 ml)

x = 2 ml


The problem is the total volume is 10 ml, hence 2 ml stock and 8 ml of sample.... but I need to use all my 10 ml. How do I know how much of the 500 mM stock to add to achieve a final concentration of 100mM?? Two variables! The amount of stock added and the total volume.

help!

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I'm not quite sure what your issue is. Is your sample already at 10mL? If so, it's still a one-variable problem. All you need do is use the term (10 + x)mL instead of 10mL for your final volume and then do the algebra. If your sample is at 8mL, then start there, add your two mLs, it's at 10mLs and you're done.

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Thanks, thats true. I think I've got it figured out.

I just am making a stock that is 100 mM EDTA and 100 mM EGTA, then solving for the original concentration stock.

(x)(2 ml)=(100mM)(10 ml)

x=600 mM

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Add 2 ml of EDTA to 6 ml of sample, then add 2 ml of EGTA to that to be at 10 ml final volume and 100 mM in each EDTA and EGTA.

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I am working with a 10 ml filtrate - the concentration must be 100mM each EDTA and EGTA in whatever total volume.

This is the correct solution as TheSquire pointed out:

(0.5M)(x)=(0.1M)(10 ml+x)

x=2.5 ml

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