Question 527956: Sorry, this was a section that I was not able to cover due to schedule changes. Thank You!
The problem: Rhonda mixes some 35% acid solution with some 70% acid solution to make 100mL of a 42% acid solution. How much of the 35% acid solution did she use?
Again, Thank You for your time,
A student
Answer by oberobic(2304)   (Show Source):
You can put this solution on YOUR website! With mixture problems, you need to determine how much 'pure' stuff you have or need.
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She needs to make 100 mL @ 42% = 42 mL of pure acid.
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The two amounts will total 100 mL, so she adds
x mL of 35% acid with
100-x mL of 70% acid
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.35x + .7(100-x) = .42(100)
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multiply both sides by 100 to eliminate decimals
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35x + 70(100-x) = 42(100)
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35x + 7000 -70x = 4200
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-35x = -2800
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x = -2800/-35 = 80 mL of 35% acid
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100-x = 20 mL of 70% acid
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check how much pure acid you have to be sure
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.35*80 = 28 mL
.70*20 = 14 mL
28+14 = 42 mL
correct
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Answer: Mix 80 mL of 35% acid + 20 mL of 70% acid to produce 100 mL of 42% acid.
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Done.
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