If you have 200.0 mL of a 30.0% aqueous solution of isopropyl alcohol, what volume of water is in the solution?
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Who knows? 30% in 200 mL solution means 60 g isopropyl alcohol with enough water added to make a total volume of 200 mL of solution. That will be 200-60 = 140 mL water IF (and ONLY IF) the volumes of isopropyl alcohol and water are additive. I expect they are not. For example, 500 mL ethyl alcohol added to 500 mL water gives a total volume very close to 950 mL (not 1000 which makes it about 50 mL short).
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