[問題]考古題Form18 sec7 DS-Q15

[凱薩琳]

A pharmacist used distilled water to dilute a solution that was 25 percent methyl alcohol by volume. If the new solution was 10 percent methyl alcohol by volume, how many milliliters of distilled water was used?

(1)The volume of the 10 percent solution was 120 milliliters.
(2)The volume of the 25 percent solution was 48 milliliters

Ans:D
這題題目好複雜，我看很久還不是很懂耶..

[StanleyHuang]

題目是說：有一個藥劑師用蒸餾水(就是純水的意思)來稀釋體積比為25%的甲醇溶液。如果稀釋完後的新溶液的體積比是10%，那到底加了多少水進去？

假設原先25%的甲醇溶液是 V (ml) ==> 純甲醇有 0.25*V (ml)
假設加進了 K (ml)的純水 ==> 新溶夜的濃度 = (0.25*V) / (V+K) = 0.1

由(1) 可得 (V+K) = 120 代進上面兩個式子可得 V 與 K
由(2) 可得 V=48 代進上面第二個式子可得 K

所以選(D)

[凱薩琳]

我懂了..感謝