SOLUTION: A flask contains 500 ml of an 80% alcohol solution in water. How much of this solution must be drawn off and replaced by water to obtain a solution having a concentration of 25% al

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Question 768499: A flask contains 500 ml of an 80% alcohol solution in water. How much of this solution must be drawn off and replaced by water to obtain a solution having a concentration of 25% alcohol?
what are the steps to solve this problem? i get confused when there are already situations where you have to replace and to draw off things in the problem. Thankyou :)
Answer by josgarithmetic(39618) About Me  (Show Source):
You can put this solution on YOUR website!
This calculation will be simpler than if you were replacing the solution with a different concentration of alcohol solution.
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You begin with a liquid of 500 ml. volume, and you finish with a liquid of 500 ml. volume. When you perform the replacement with the water, you are adding NO alcohol.

Let v = volume in milliliters to remove, before replacing with the v of water.
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highlight%28%28500%2A%280.8%29-v%280.8%29%2Bv%2A%280%29%29%2F500=0.25%29
This is using decimal fractions, being my attempt to make this seem less confusing (maybe it works, maybe not).
The equation is in this case, perfectly equivalent to
highlight%28%28500%2A%280.8%29-v%280.8%29%29%2F500=0.25%29
or if desired, equivalent to
highlight%28%280.8%28500-v%29%29%2F500=0.25%29

SOLVE FOR v.


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Do the arithmetic and find v = 343.75 ml. but in practice it should really be 343 or 344.