Combining different solution strengths

by Preetika

While organizing stock, the pharmacy manager finds 3 different bottles of isopropyl alcohol. She asks you to measure, consolidate, and re-label them into the largest container, which is a gallon bottle containing 1.5L of 90%. The second bottle contains 1L of 70%. The third bottle contained 1.25L of 70%. What will you write on the new label?

A. 74%-Isopropyl Alcohol
B. 81%-Isopropyl Alcohol
C. 86%-Isopropyl Alcohol
D. 78%-Isopropyl Alcohol

I don't even know where to begin with this. Please help. The answer is supposed to be 78%

Comments for Combining different solution strengths

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IPA concentration problem
by: Brad Wojcik, Pharm D

To calculate the final concentration you will need to know the amount of IPA in the final solution along with the total volume.
You have:
1.5 L 90% = 1.35 L IPA
1 L 70% = 0.7 L IPA
1.25 L 70% = 0.875 L IPA

Now add up both sides:

Total volume = 1.5 L + 1 L + 1.25 L = 3.75 L
Total volume of IPA = 1.35 L + 0.7 L + 0.875 L = 2.925 L
Now change that to a percent: (2.925 L/3.75 L)(100%) =78%

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