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Let the amount of pure alcohol be x, so the amount of 25% solution would be x + 5. So we have:

x + 0.15(5) = 0.25(x + 5)

x + 0.75 = 0.25x + 1.25

x - 0.25x = 1.25 - 0.75

0.75x = 0.5

x = 0.5/0.75 = 0.666... = 6/9 = 1/3 of the liter

Added note

to deal with the so called 'dilution contraction' of total volume

If it were % by MASS ( %m/m), it's quite easy to do (based on the 'Mass Conservation Law). You calculate with mass (kg) and mass-% (%m/m) i.s.o. volume (L) and vol% (%v/v).

However if the meaning was: % by Volume ( %v/v) then calculation appears to become quite complicated, but not impossible if you know at least the density values of all solutions (original 100%v/v or 15%v/v and final 25%v/v).

DO NOT use: (orig. volume) + (added volume) = final volume, as done above, if exact figures are necessary.

It's only a rule of thump, an approximation. This is because fluids can contract on mixing at dilution. There is no rule such as: conservation of volume.

Your case: 0.33 L + 5 L (is not equal but) < 5.33 L final solution.

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11y ago
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12y ago

15 L of 60% & 25 L of 20%. Here's how to solve:

Let A = volume of 60% solution, and B = volume of 20% solution, then:

  1. Volume of solute in A is 0.6*A
  2. Volume of solute in B is 0.2*B
  3. Volume of final solute is 0.35*(final total volume), so:
  • Total vol is 40: A + B = 40
  • Solute is 0.35*40 = 14: 0.6*A + 0.2*B = 14.

Two equations and two unknowns: A = 15, B = 25

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13y ago

You would use 100/3 = 33.33 gallons of the 40 percent antifreeze solution and 50/3 = 16.67 gallons of the 10 percent antifreeze solution.

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Q: How many gallons of a 40 percent antifreeze solution should be mixed with a 10 percent solution to make 50 gallons of a 30 percent solution?
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How many gallons of a 50 percent antifreeze solution must be mixed with 90 gallons of 30 percent antifreeze to get a mixture that is 40 percent antifreeze?

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