The ethanol content of alcoholic beverages is sometimes expressed in terms of “p

Question

The ethanol content of alcoholic beverages is sometimes expressed in terms of “proof.” This term comes from a 17th century test for the alcohol content in whiskey. The whiskey was poured onto gunpowder and then set on fire. If the whiskey was too wet, the gunpowder would not ignite after the whiskey had burned off. However, if the whiskey had not been watered down, the gunpowder would ignite. A positive test required a minimum ethanol content of approximately 50% ethanol (by volume), which was called “100 proof.” Whiskey with 40% ethanol is “80 proof,” and so on.

What is the approximate molar concentration of ethanol in 100 proof whiskey? The density of pure ethanol is 0.789 g mL-1. Report your answer to two significant figures.

If the vapour pressure of water is 17.5 torr at 20°C, what is the water vapour pressure inside a bottle of 100 proof whiskey?

Explanation / Answer

volume % of ethanol in 100 proof = 50 %

denisty = 0.789 g/ml

mass of ethanol = density x volume = 50 x 0.789

                           = 39.45 g

moles of ethanol = 39.45 / 46 = 0.858

Molarity = moles / volume (L)

              = 0.858 / 0.1

              = 8.58 M

molar concentration = 8.58 M

moles of water = 50 / 18 = 2.78

mole fraction of ethanol = 0.858 / (0.858 + 2.78)

                                      = 0.236

Po - Ps / Po = X ethanol

17.5 - Ps / 17.5 = 0.236

Ps = 13.37 torr

vapour pressure = 13.4 torr

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