Question

How much water should be mixed with 12ml of alcohol so we can obtain 12% alcohol solution

Answer 1

Your best friend in science is the C1V1=C2V2 equation.

This says that the concentration you begin with, multiplied by the volume you use of it, will be equal to the final concentration, multiplied by whatever the final volume is.

For a simple example, if you were to dilute something in half, you might take 1ml of 50% ethanol and put it into 1ml of water, leaving the final volume at 2ml. How do we get the final concentration? Solve the equation for C2 and plug in the values:

C2=C1V1V2

C2=(1ml)(0.5)2ml

So, C2=0.25 or 25%.

In your case, you would solve for V1, plug in 100ml as your final volume (V2), 30% - or 0.3 - as your final concentration (C2), and 95% - or 0.95 - as your initial concentration (C1). This will give you the volume, of your starting ethanol mixture, to add to a total volume of 100ml, to obtain a 30% solution.