Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Watery on January 16, 2015, 10:15:11 AM

Hello. There's this question that's been giving me trouble. It's a pretty question and I'm a bit frustrated that I can't figure it out because I haven't taken gen chem in a while. But here it is
Commercial windshield washer fluid is 60% methanol and 40% water by weight. What is the
percentage by volume of methanol in 11.0 g of windshield washer fluid? The density of the
mixture is 0.896 g/mL.
I believe it's a stoichiometry problem. So I multiplied 0.896 x 0.6 as the answer is asking for only methanol. But I am not sure what to do next. Please help

There's no reaction, so stoichiometry really doesn't apply. You're given a weight:weight ratio, but you have volume. How will you convert? Hint: you've been given density.

There's no reaction, so stoichiometry really doesn't apply. You're given a weight:weight ratio, but you have volume. How will you convert? Hint: you've been given density.
Oh right
I'm guessing I would multiply the 10 grams by 60% for methanol and 10 grams by 40% methanol to get 6 and 4 respectively. Then I would multiply 6 by the density (0.896) and the 4 by 0.896 to cancel the grams and give me volumes for both. Then I would add those volumes and (5.376 grams for the methanol) and (3.584 for water) to get a total of 8.96. Then since it's asking for methanol, do 5.376/8.96 to get 60% volume for methanol. But that doesn't seem right at all. I think my second step is incorrect

There is not enough data to solve the problem  unless you are expected to assume water density of 1 g/mL.
Note that the question
What is the percentage by volume of methanol in 11.0 g of windshield washer fluid?
is in a way funny  percentage by volume doesn't depend and the amount of the liquid.
I would start with 10 g. What is the volume? How much of that is occupied by water (assuming water density listed earlier)?