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### AuthorTopic: [1] Mass of urea, given mass of solution and mole fraction of urea  (Read 4533 times) !function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0];if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src="https://platform.twitter.com/widgets.js";fjs.parentNode.insertBefore(js,fjs);}}(document,"script","twitter-wjs"); (function() {var po = document.createElement("script"); po.type = "text/javascript"; po.async = true;po.src = "https://apis.google.com/js/plusone.js";var s = document.getElementsByTagName("script")[0]; s.parentNode.insertBefore(po, s);})();

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#### Gobo

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##### [1] Mass of urea, given mass of solution and mole fraction of urea
« on: January 30, 2012, 04:30:59 AM »

Find the mass of urea (CH4N2O) needed to prepare 51.8g of a solution in water in which the mole fraction of urea is 7.55×10−2.

Is this a valid assumption?
The molar mass of the solution is MM(H2O) + MM(NH4N2O)

So then, MM(sol'n) = (18.02 + 60.062)g/mol sol'n = 78.082g/mol sol'n
-------------------------------------------------------------------------------------------------------------------

If the mole fraction of urea is 0.0755, then the mole fraction is water is 0.9245.
I would like a hint, and know if my assumption will work.
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#### sjb

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##### Re: [1] Mass of urea, given mass of solution and mole fraction of urea
« Reply #1 on: January 30, 2012, 05:32:33 AM »

Find the mass of urea (CH4N2O) needed to prepare 51.8g of a solution in water in which the mole fraction of urea is 7.55×10−2.

Is this a valid assumption?
The molar mass of the solution is MM(H2O) + MM(NH4N2O)

So then, MM(sol'n) = (18.02 + 60.062)g/mol sol'n = 78.082g/mol sol'n
-------------------------------------------------------------------------------------------------------------------

If the mole fraction of urea is 0.0755, then the mole fraction is water is 0.9245.
I would like a hint, and know if my assumption will work.

First stab... not really, no.

What would the mass of 0.9245 mol of water be, and of 0.0755 mol of urea? What is the total mass? So what factor would you need to scale by to get a total solution of 51.8g...
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#### Gobo

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##### Re: [1] Mass of urea, given mass of solution and mole fraction of urea
« Reply #2 on: January 30, 2012, 05:51:20 AM »

Find the mass of urea (CH4N2O) needed to prepare 51.8g of a solution in water in which the mole fraction of urea is 7.55×10−2.

Is this a valid assumption?
The molar mass of the solution is MM(H2O) + MM(NH4N2O)

So then, MM(sol'n) = (18.02 + 60.062)g/mol sol'n = 78.082g/mol sol'n
-------------------------------------------------------------------------------------------------------------------

If the mole fraction of urea is 0.0755, then the mole fraction is water is 0.9245.
I would like a hint, and know if my assumption will work.

First stab... not really, no.

What would the mass of 0.9245 mol of water be, and of 0.0755 mol of urea? What is the total mass? So what factor would you need to scale by to get a total solution of 51.8g...

OH! So we could say per 1 mole of solution, we have 0.9245mol of H2O, and 0.0755 mol of urea, just like how I did above?
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#### Borek

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##### Re: [1] Mass of urea, given mass of solution and mole fraction of urea
« Reply #3 on: January 30, 2012, 05:51:52 AM »

The molar mass of the solution is MM(H2O) + MM(NH4N2O)

No such thing as a 'molar mass of the solution'. Solution is a mixture, molar mass is a property of a single substance.
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#### Gobo

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##### Re: [1] Mass of urea, given mass of solution and mole fraction of urea
« Reply #4 on: January 30, 2012, 06:18:29 AM »

The molar mass of the solution is MM(H2O) + MM(NH4N2O)

No such thing as a 'molar mass of the solution'. Solution is a mixture, molar mass is a property of a single substance.

Thank you. That clears any subsequent attempts for that approach.

Quote
OH! So we could say per 1 mole of solution, we have 0.9245mol of H2O, and 0.0755 mol of urea, just like how I did above?
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