Question

Rubbing alcohol is typically diluted with water to 70% strength. if you need 3.5 oz of 45% rubbing alcohol, how many ounces of 70% rubbing alcohol and how much water should you combine?

Answer 1

1.25 oz. You must combine 1.25 oz water with 2.25 oz 70 % rubbing alcohol (RA) to make 3.5 oz of 45 % RA .

Step 1. Calculate the volume of 70 % RA 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 = 3.5 oz; C1 = 45 %

V2 = ?; ___C2 = 70 %

∴V2 = 3.5 oz × (45 %/70 %) = 2.25 oz

You need 2.25 oz of 70 % RA to make 3.5 oz of 45 % RA

Step 2. Calculate the amount of water to add.

Volume of water = 3.5 oz - 2.25 oz = 1.25 oz