Question

You need 270 ml of a 65% alcohol solution. on hand, you have a 90% alcohol mixture. how much of the 90% alcohol mixture and pure water will you need to obtain the desired solution?

Answer 1

You must add 75 mL water to 195 mL 90 % alcohol to make 270 mL of 65 % alcohol.

Step 1. Calculate the volume of 90 % alcohol needed

You can use the dilution formula

V1×C1 = V2×C2

where

V1 and V2 are the volumes of the two solutions

C1 and C2 are the concentrations

You can solve the above formula to get

V2 = V1 × C1/C2

V1 = 270 mL; C1 = 65 %

V2 = ?; _____C2 = 90 %

∴V2 = 270 mL × (65 %/90 %) = 195 mL

You need 195 mL of 90 % alcohol to make 270 mL of 65 % RA

Step 2. Calculate the amount of water to add.

Volume of water = 270 mL – 195 mL = 75 mL