# Chemical Forums

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

Title: Help with this question? {percentage by weight}
Post 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
Title: Re: Help with this question? {percentage by weight}
Post by: Arkcon on January 16, 2015, 10:20:27 AM
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.
Title: Re: Help with this question? {percentage by weight}
Post by: Watery on January 16, 2015, 10:39:09 AM
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
Title: Re: Help with this question? {percentage by weight}
Post by: Borek on January 16, 2015, 12:24:16 PM
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)?