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November 22nd, 2016, 10:26 AM   #1
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help solve problem

I have 30ml solution of 0.05%. How much water needed to add to make a solution of 0.04%?
Once I have solution of 0.04%, how much water needed to add to make a solution of 0.03%?
Thank you.

Last edited by skipjack; November 22nd, 2016 at 12:08 PM.
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November 22nd, 2016, 10:59 AM   #2
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x = mL of pure water

30(.05) + x(0) = (30+x)(.04)

solve for x, then substitute the calculated value for x in the following equation ...

y = mL of pure water

(30+x)(.04) + y(0) = (30+x+y)(.03)

solve for y
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November 22nd, 2016, 12:22 PM   #3
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Is the original solution 0.05% or 5%?
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November 22nd, 2016, 12:29 PM   #4
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Quote:
Originally Posted by skipjack View Post
Is the original solution 0.05% or 5%?
doesn't matter as long as the other given concentrations are relatively the same ... true?
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November 22nd, 2016, 03:52 PM   #5
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It's this simple (w = water):

.....30 @ 5
......w @ 0
=======
30+w @ 4

(30*5 + w*0) / (30 + w) = 4
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