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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: goosefraba on September 22, 2010, 09:03:19 PM

Title: finding mole fraction from percent by weight
Post by: goosefraba on September 22, 2010, 09:03:19 PM

What is the mole fraction of water in a solution that is 37.9% by weight ethylene glycol? The molar mass of ethylene glycol is 62.07g/mol.

Please!

Thanks

Title: Re: finding mole fraction from percent by weight
Post by: opti384 on September 23, 2010, 02:17:15 AM

The mole fraction will be

the mols of water (in this case) / total mols 

Title: Re: finding mole fraction from percent by weight
Post by: jena2nsu on June 10, 2012, 05:17:33 PM

So, I'm trying to work this problem with little success. I have the molar mass of H2O as 18.00988 and the molar mass of HOCH2CH2OH as 62.07 g/mol. Using this information, I do the problem as.... but can't seem to get the desired answer.

62.07+18.00988=80.07988

62.1 g/mol / 80.07988 = 0.7757   <---- This is not the correct answer. How am I doing this problem wrong?

The correct answer, according to the answer sheet is 0.849.

Please help me figure this out!

Title: Re: finding mole fraction from percent by weight
Post by: Borek on June 10, 2012, 05:22:12 PM

Start assuming you have 1000 g of the mixture (or any other mass, it doesn't matter).

How many grams of water in the mixture?

How many moles is that?

And ethylene glycol?

Title: Re: finding mole fraction from percent by weight
Post by: DrCMS on June 10, 2012, 05:24:43 PM

Quote from: jena2nsu on June 10, 2012, 05:17:33 PM

So, I'm trying to work this problem with little success. I have the molar mass of H2O as 18.00988 and the molar mass of HOCH2CH2OH as 62.07 g/mol.

So far so good.

Quote from: jena2nsu on June 10, 2012, 05:17:33 PM

Using this information, I do the problem as.... but can't seem to get the desired answer.

62.07+18.00988=80.07988

62.1 g/mol / 80.07988 = 0.7757   <---- This is not the correct answer. How am I doing this problem wrong?

You are going wrong by not using the weight % given in the question.

Take an imaginary 100g of this solution.
How many grams of water and how many of glycol?
How many moles of each?
Now redo the mole fraction.

Edit - Borek beat me to it.

Title: Re: finding mole fraction from percent by weight
Post by: jena2nsu on June 10, 2012, 05:53:13 PM

Take an imaginary 100g of this solution.
How many grams of water and how many of glycol?
How many moles of each?
Now redo the mole fraction.

I'm still not getting the desired answer. I'm using 62.1g H2O and 37.9 g HOCH2CH2OH. I'm using molar masses of 62.07 HOCH2CH2OH and 18.01528 H2O. The combined molar mass is 80.08528.

Because we want to measure the mole fraction of H20 which represents 62.1 g of the solution, I complete the problem as:

62.1 g/80.08528 = .775423

I still can't figure out what I'm doing wrong after following the provided directions. Can someone type out the portion I'm getting wrong to help me see. I have all the other problems correct except this one equation.

Title: Re: finding mole fraction from percent by weight
Post by: jena2nsu on June 10, 2012, 05:57:32 PM

Nevermind! I got the equation! The grams had to be converted to moles.

3.447 m of H2O
.61050 m of HOCH2CH2OH

All added up to be 4.06 m of solution.

I then divided 3.45 by the total solution of 4.06 which equals the desired answer of .84975.

Thanks guys! I figured it out!