Solution: The term proof is defined as twice the percent by volume of pure ethanol in solution. Thus, a solution that is 95% (by volume) ethanol is 190 proof. What is the molarity of ethanol in a 92 proof ethanol–water solution? Assume the density of ethanol, C2H5OH, is 0.79 g/cm3 and the density of water is 1.0 g/cm3.

Problem

The term proof is defined as twice the percent by volume of pure ethanol in solution. Thus, a solution that is 95% (by volume) ethanol is 190 proof. What is the molarity of ethanol in a 92 proof ethanol–water solution? Assume the density of ethanol, C2H5OH, is 0.79 g/cm3 and the density of water is 1.0 g/cm3.

Solution

First, let us establish that ethanol will be referred to as EtOH throughout the calculations as shorthand.

Now the relationship of proof to %volume can be expressed by the equation below:

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