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#### Coastie17

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##### Calculating Mole Fraction
« on: November 24, 2010, 03:15:17 PM »

I am having some difficulty with this question. I feel like more information is needed. Any help would be greatly appreciated! Thanks!

A substance is composed of 17% oxygen, 50% helium and the rest is nitrogen. Assuming these are mass percents, calculate the mole fraction of each gas in the substance.

Here's what I came up with:

mass of O = 17% x 34.01 g (molar mass of all atoms) = 5.78 g O (1 mol O/16.00 g O) = 0.361 mol O
mass of He = 50% x 34.01 g = 17.0 g He (1 mol He/4.003 g He) = 4.25 mol He
mass of N = 33% x 34.01 g = 11 g N (1 mol N/14.01 g N) = 0.79 mol N
total mol = 0.361 + 4.25 + 11. = 5.4 mol

mol fraction O = 0.361 mol/5.4 mol = 0.067
mol fraction He = 4.25 mol/5.4 mol = 0.787
mol fraction N = 0.79 mol/5.4 mol = 0.15
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#### Borek

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##### Re: Calculating Mole Fraction
« Reply #1 on: November 24, 2010, 09:25:32 PM »

Your result seems to be - accidentally - correct, even if the mass you have used (34.01 g) doesn't make any sense. Trick is, it cancels out.

Imagine you have a 100g sample of the substance. Obviously it contains 17g of oxygen, 50g of helium and 33g of nitrogen. Convert to moles - and you are ready to calculate molar fractions.

Imagine you have a 1000g sample of the substance. Obviously it contains 170g of oxygen, 500g of helium and 330g of nitrogen. Convert to moles - and you are ready to calculate molar fractions.

Imagine you have ANY sample of the substance. Obviously you can calculate how much oxygen, helium and nitrogen it contains. Convert to moles - and you are ready to calculate molar fractions.

Say you have sample of mass m.

Molar fraction of oxygen is

${ \frac {\frac {0.17 \times m}{16}} {\frac {0.17 \times m}{16} + \frac {0.50 \times m}{4} + \frac {0.33 \times m}{14}} }$

- do you see that m cancels out?
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#### DrCMS

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##### Re: Calculating Mole Fraction
« Reply #2 on: November 24, 2010, 10:26:54 PM »

The answers are not correct as oxygen gas is O2 and nitrogen gas is N2
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#### Borek

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##### Re: Calculating Mole Fraction
« Reply #3 on: November 25, 2010, 03:15:42 AM »

The answers are not correct as oxygen gas is O2 and nitrogen gas is N2

I guess the problem is in nomenclature. If substance means "mixture of gases", you are right. If substance is a compound, I would go for 1 atom 1 mole.

What is molar fraction of hydrogen in H2S? Is it 2/3, or 1/2, or 8/9?
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#### DrCMS

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##### Re: Calculating Mole Fraction
« Reply #4 on: November 25, 2010, 06:40:14 AM »

Assuming these are mass percents, calculate the mole fraction of each gas in the substance.

I'd say based on the term "mole fraction of each gas in the substance" the substance is a mixture of gases rather than a molecule.

What is molar fraction of hydrogen in H2S? Is it 2/3, or 1/2, or 8/9?

I'd say 2/3
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#### Borek

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##### Re: Calculating Mole Fraction
« Reply #5 on: November 25, 2010, 10:37:19 AM »

What is molar fraction of hydrogen in H2S? Is it 2/3, or 1/2, or 8/9?

I'd say 2/3

So the answers were right, unless I am missing something big.
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#### DrCMS

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##### Re: Calculating Mole Fraction
« Reply #6 on: November 26, 2010, 07:31:39 AM »

What is molar fraction of hydrogen in H2S? Is it 2/3, or 1/2, or 8/9?

I'd say 2/3

So the answers were right, unless I am missing something big.

Yes that the questions asks for the mole fraction of each gas rather than each element

The question is about a mixture of gases,  17% O2 33%N2 and 50%He.

Besides do you know any molecule that is 17% oxygen, 50% helium 33% nitrogen by mass percents?
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#### ardbeg

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##### Re: Calculating Mole Fraction
« Reply #7 on: November 28, 2010, 02:14:22 PM »

the question says substance which to me doesnt mean mixture.  badly worded question.
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#### DrCMS

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##### Re: Calculating Mole Fraction
« Reply #8 on: November 28, 2010, 11:00:11 PM »

the question says substance which to me doesnt mean mixture.  badly worded question.

I agree it is a badly worded question but I think the only answer that makes any chemical sense assumes it is a gas mixture.

If as you say the substance is a compound the answer is He188N35O16. Does that look like a real compound to you?
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#### rabolisk

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##### Re: Calculating Mole Fraction
« Reply #9 on: November 29, 2010, 02:54:15 AM »

Clearly the "substance" could not be any compound since it is 50% helium.
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#### Borek

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##### Re: Calculating Mole Fraction
« Reply #10 on: November 29, 2010, 10:52:45 AM »

We are all guessing. Just because question asks about impossible compound doesn't mean it asks about mixture, as "substance" suggests something else. And after seeing thousands of stupid questions (and even more stupid answers; I have whole collection of these) I would not dare to say which approach is correct. Unfortunately questions that give impossible formulas are quite common, and often students are expected to shut up and calculate, even if the logic shows it doesn't make sense.
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