Calculating Molarity of Ethyl Alcohol in Whiskey

[Jim4592]

Homework Statement

Eighty-six proof whiskey is 43 percent ethyl alcohol, CH₃CH₂OH, by volume. If the density of ethyl alcohol is 0.79 kg/L, what is the molarity in whiskey.

Homework Equations

Molar Mass of CH₃CH₂OH = 46.07 g

The Attempt at a Solution

0.79 kg/L * 1000 g/kg * 1 mole / 46.07 g = 17.15 M

I was just looking for a check on this particular problem since I haven't taken a chemistry course since my freshman year ha!

 

[Borek]

Not bad - you are on the right track - but wrong. You have not used 43% in your calculations and this is an important information.

 

[Jim4592]

I thought you would have to use that 43% in there somewhere, but I'm not sure how to use it. It would be nice if I still owned my chem book.

UPDATE:

Ok I tried re-working the problem again, here's what I came up with:

0.79 kg/L * 0.43 L / 1 L * 1000 g / 1 kg * 1 mol / 46.07 g = 7.37 M

how does that look?

 

[Borek]

Much better.

 

[DeeNos]

Your calculation looks correct. Molarity is a measure of the concentration of a solution, and it is determined by dividing the moles of solute (in this case, ethyl alcohol) by the volume of the solution in liters. In this problem, you correctly converted the density of ethyl alcohol to mass (kg) and then to moles, using the molar mass of ethyl alcohol. This gave you the molarity of ethyl alcohol in whiskey, which is 17.15 M. Good job!