How much of the 85% alcohol mixture and pure water will you need to obtain the desired solution?

Question

You need 255 mL of a 45% alcohol solution. On hand, you have a 85% alcohol mixture. How much of the 85% alcohol mixture and pure water will you need to obtain the desired solution?

Answer 1

x = amount of pure water

255 - x = amount of 85%

 

Since pure water is being added the solutions must be presented in % of water.

 

0.15(255 - x) + x = 0.55%(255)    85% alcohol is 15% water, 45% alcohol is 55% water.

38.25 - 0.15x + x = 140.25

0.85x = 10.20

x = 120

 

One hundred twenty milliliters of water should be added to one hundred thirty-five milliliters of the 85% solution.