Hello, Yesenia,
An amount of liquid is not specified, so let's assume a convenient number: 1 liter. It makes no difference as to the volume we pick as a starting point. 1 ml or the Pacific ocean - it doesn't matter. The information above simply says 20% of SOMETHING (amount). We'll pick 1 liter.
ABC = alcohol by volume, so 20% of 1 liter is 200 ml.
The alcohol density is 0.789 g/ml.
Multiply the two to get grams of alcohol: 200 ml x 0.789 g/ml = 157.8 grams alcohol.
The molar mass of C2H6O is 46 g/mole.
To calculate the total moles, divide the alcohol mass by its molar mass : 157.8 g /46 g/mole = 3.43 moles alcohol. (The grams cancel and the moles moves to the top).
We assumed 1 liter total solution. Molarity is defined as "moles/liter. So 3.43 moles/1 liter = 3.43 M.
[That';s why I picked 1 liter to start - makes it easy. But it will work with whatever initial volume you prefer to select.]
Technically, there is only 1 sig fig ( the 20%), so the correct answer would need to be rounded to 3 M. Painful.
I hope this helps,
Bob
