For your recrystallization, you will need to make 75% ethanol in water from 95% ethanol in water. Calculate how much 95% ethanol and water you will need to make 20 mL of 75% ethanol. Hint: You may want to use M1V1=M2V2.

15.79 mL of ethanol and 4.21 mL of water

Explanation:

Initial concentration of ethanol = 95%

Final concentration of ethanol (to be prepared) = 75%

Initial volume of ethanol to be taken = ?

Final volume of ethanol to be prepared = 20 ml

Using the dilution equation: M1V1 = M2V2, where;

M1 = initial concentration = 95%

V1 = initial volume = ?

M2 = final concentration = 75%

V2 = final volume = 20 mL

Substitute each value in the equation:

95 x (initial volume) = 75 x 20

Initial volume = 75 x 20/95

= 15.79 mL

The volume of 95% ethanol to be taken is 15.79 mL.

In order to calculate the volume of water need, subtract the initial ethanol volume from the final volume of solution to be prepared.

20 - 15.79 = 4.21 mL

Hence, 15.79 mL of 95% ethanol and 4.21 mL of water will be needed to make 20 mL of 75% ethanol.