Question

You need a 15% alcohol solution. On hand, you have a 120 mL of a 70% alcohol mixture. How much pure water will you need to add to obtain the desired solution? A) Write an equation using the information as it is given above that can be used to solve this problem. Use x as your variable to represent the amount of pure water you need to use. Equation: B) You will need mL of pure water to obtain mL of the desired 15% solution.

Answer 1

We need a 15% solution

We have 120 ml of a 70% solution

And we want to find the amount of water to obtain the final concentration

x as your variable to represent the amount of pure water you need to use.

Part A

We have at the begin 120 ml with 70% alcohol and 30 % water

And we want to obtain a 15% alcohol solution

x = amount of water to be added:

Then we can create the following balance:

Alcohol: 0.7 *120 + 0*x = 0.15(120+x)

84 = 18 + 0.15 x

Part B

Solving for x we got:

84 = 0.15x+ 18

x= (84-18)/0.15 = 440 ml

You will need:

440 ml of pure water

to obtain

560 ml of the desired 15% solution