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# Desperate help with chemistry questions mole fractions watch

1. a). A flask contains 2.00 mol nitrogen and 2.00 mol helium. How many grams of Argon must be pumped into the flask to make the partial pressure of argon twice that of helium.

So the way I attempted this was put x as moles of argon. so the mole fraction of helium would be 2/(2+2+x) and argon needs to be double the mole fraction so 2*x/(2+2+x).
So I set up the equation 2/(4+x) = 2x/(4+x) I solved this to find x = 1 so I thought this meant theres 1 mol of argon and this mass = 39.9 However the answer is 160 -so this is obviously an incorrect method - can someone help me?

b). A mixture of 14.0 g hydrogen, 84.0 g nitrogen and 2.00 mol oxygen are placed in a flask. When the partial pressure of oxygen is 0.102 atm, what is the total pressure in the flask?

For this question i found the mole fraction of O2 to be 2/22 and this multiplied by x (total pressure) will give the partial pressure of oxygen 0.102 atm. So I did 0.102/0.0909... to get total pressure as 1.122 however this is also wrong because the answer should be 0.612 atm. Any help would be very much appreciated thankyou
2. For a) you will need twice as many moles of argon as you have of helium. As you have no argon in the flask initially you will need to add 4 moles of argon, approx 160g.
3. (Original post by TutorsChemistry)
For a) you will need twice as many moles of argon as you have of helium. As you have no argon in the flask initially you will need to add 4 moles of argon, approx 160g.
Is that how simple it really is?? What about part b
4. For b) how many moles of hydrogen and of nitrogen did you calculate to be present?
5. (Original post by TutorsChemistry)
For b) how many moles of hydrogen and of nitrogen did you calculate to be present?
14 moles of hydrogen and 6 moles of nitrogen...
6. (Original post by MrToodles4)
14 moles of hydrogen and 6 moles of nitrogen...
I think you may have made a very common error.

Hydrogen doesn't exist as H, but as H2. Hydrogen gas has molar mass of 2, so there are 7 moles.
Similarly, nitrogen exists as N2. Nitrogen gas has molar mass of 28, so there are 3 moles of nitrogen.
7. Hydrogen, nitrogen and oxygen are diatomic molecules.
8. (Original post by MrToodles4)
a). A flask contains 2.00 mol nitrogen and 2.00 mol helium. How many grams of Argon must be pumped into the flask to make the partial pressure of argon twice that of helium.

So the way I attempted this was put x as moles of argon. so the mole fraction of helium would be 2/(2+2+x) and argon needs to be double the mole fraction so 2*x/(2+2+x).
So I set up the equation 2/(4+x) = 2x/(4+x) I solved this to find x = 1 so I thought this meant theres 1 mol of argon and this mass = 39.9 However the answer is 160 -so this is obviously an incorrect method - can someone help me?

b). A mixture of 14.0 g hydrogen, 84.0 g nitrogen and 2.00 mol oxygen are placed in a flask. When the partial pressure of oxygen is 0.102 atm, what is the total pressure in the flask?

For this question i found the mole fraction of O2 to be 2/22 and this multiplied by x (total pressure) will give the partial pressure of oxygen 0.102 atm. So I did 0.102/0.0909... to get total pressure as 1.122 however this is also wrong because the answer should be 0.612 atm. Any help would be very much appreciated thankyou
For part a, you may have got the wrong answer but the idea behind your method is good, and although here it wasn't necessary to set up the problem mathematically, it is a very useful skill for more difficult problems, so don't be disheartened! you only went wrong because you made one small mistake!

Mole fraction of Helium is 2/(2+2+x) as you said

The mole fraction of argon however is simply x/(2+2+x)

the question wants the mole fraction of argon to be equal to double the mole fraction of helium, so

x/(4+x) = 4/(4+x)

clearly the solution is then x=4 and you get the right answer.

Here it was easier to realise that mole fraction is proportional to number of moles, so twice the moles gives twice the mole fraction, but your method was good
9. (Original post by MexicanKeith)
For part a, you may have got the wrong answer but the idea behind your method is good, and although here it wasn't necessary to set up the problem mathematically, it is a very useful skill for more difficult problems, so don't be disheartened! you only went wrong because you made one small mistake!

Mole fraction of Helium is 2/(2+2+x) as you said

The mole fraction of argon however is simply x/(2+2+x)

the question wants the mole fraction of argon to be equal to double the mole fraction of helium, so

x/(4+x) = 4/(4+x)

clearly the solution is then x=4 and you get the right answer.

Here it was easier to realise that mole fraction is proportional to number of moles, so twice the moles gives twice the mole fraction, but your method was good
Thank you so so much! I understand it now

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Updated: October 15, 2017
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