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
.
<em>Step 1.</em> Calculate the volume of 70 % RA needed
You can use the dilution formula
<em>V</em>1×<em>C</em>1 = <em>V</em>2×<em>C</em>2
where
<em>V</em>1 and <em>V</em>2 are the volumes of the two solutions
<em>C</em>1 and <em>C</em>2 are the concentrations
You can solve the above formula to get
<em>V</em>2 = <em>V</em>1 × <em>C</em>1/<em>C</em>2
<em>V</em>1 = 3.5 oz; <em>C</em>1 = 45 %
<em>V</em>2 = ?; ___<em>C</em>2 = 70 %
∴<em>V</em>2 = 3.5 oz × (45 %/70 %) = 2.25 oz
You need 2.25 oz of 70 % RA to make 3.5 oz of 45 % RA
<em>Step 2.</em> Calculate the amount of water to add.
Volume of water = 3.5 oz - 2.25 oz = 1.25 oz