Question

A sample of beer is 4.6 % by mass C2H5OH. The density of beer is 0.93 g/ml. What is the molar concentration of C2H5OH in this sample of beer?
a. 9.7 x 10-4
b. 1.13 x 10-4
c. 0.975 M
d. 1.13 M

I did 0.046x930 g/L =42.78 mass alcohol

42.78/molar mass of C2H5OH (46.068) = 0.928

however, this does not match up with any of the options given, is there something I am doing wrong? Thank you!

Answer 1

mass % = g solute/100 g sample.
M = mols/L solution.

So you have 4.6 g C2H5OH/100 g solution.
molar mass C2H5OH = 46
mols C2H5OH in 4.6 g is 4.6/46 = 0.1 mol.
What is the volume? That's mass = volume x density or volume = mass/density = 100 g/0.93 = 108 mL or 0.108 L. Then M = 0.1/0.108 = about 0.93 M. The closest answer choice is c and I would go with that. My answers is more accurate but profs often give answers a little off from the correct one in what I call multiple guess answers so as to keep students guessing. :-).