Given,
Volume of solute (alcohol) = 12 mL
Let the volume of water (solvent) required be x mL.
So the volume of solution = (12 + x) mL
We know that,
Volume by volume percentage of solution (v/v%) =Volume of soluteVolume of solution×100
⇒12=1212 + x×100
⇒ 1200 = 12(12+x)
⇒ 1200 = 144 + 12x
⇒ 12x = 1200 - 144 = 1056
⇒ x = 1056/12 = 88
Therefore, 88 mL of water should be mixed with 12 mL of alcohol to make a 12% of v/v% solution.