March 28, 2024, 10:21:06 AM
Forum Rules: Read This Before Posting


Topic: Ideal Gas law problem; determining molecular formula  (Read 6516 times)

0 Members and 1 Guest are viewing this topic.

Offline Link The Worlds

  • New Member
  • **
  • Posts: 8
  • Mole Snacks: +0/-0
Ideal Gas law problem; determining molecular formula
« on: September 04, 2010, 03:21:30 PM »
Can anyone help me with this problem?

Lets conduct an experiment to determine the molecular formula for gaseous phosphorus (determine the value of j in Pj) 893.0 g of solid phosphorus is placed in a 5.00 L container and the air in the container is then pumped out. The system is heated and the phosphorus completely sublimates. At a temperature of 450 C, the pressure is found to be 120 torr. Determine the molecular formula under these conditions.

The problem gives you all the components (I think) in the ideal gas law pV=nRT. We have p, V, n, and T. I'm not sure about n. But when i put the equation together, both sides don't equal each other.

My R constant is .0831447 L x bar/mol x K

Online Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Ideal Gas law problem; determining molecular formula
« Reply #1 on: September 04, 2010, 03:37:35 PM »
Use information given to calculate number of moles of gas.

How many moles of P are present?

How many moles of P per one mole of gas?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Link The Worlds

  • New Member
  • **
  • Posts: 8
  • Mole Snacks: +0/-0
Re: Ideal Gas law problem; determining molecular formula
« Reply #2 on: September 04, 2010, 05:23:40 PM »
Use information given to calculate number of moles of gas.

How many moles of P are present?

How many moles of P per one mole of gas?

If I use the 893 grams and look on the periodic table for P's weight (which is 30ish), I calculate that there are 28.83 moles of P.

If I use pV=nRT to solve for n, then I find that there's .013308933 moles of P.

These are contradicting each other and clearly doesn't make sense.

Secondly, aren't there 1 moles of P per one mole of P gas? This seems obvious but I'm probably wrong about it.

Online Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Ideal Gas law problem; determining molecular formula
« Reply #3 on: September 04, 2010, 06:15:31 PM »
If I use the 893 grams and look on the periodic table for P's weight (which is 30ish), I calculate that there are 28.83 moles of P.

OK

Quote
If I use pV=nRT to solve for n, then I find that there's .013308933 moles of P.

OK

Quote
These are contradicting each other and clearly doesn't make sense.

Yes and no. There is something wrong with the data, so the numbers don't make sense, but for reason other than you think.

Quote
Secondly, aren't there 1 moles of P per one mole of P gas? This seems obvious but I'm probably wrong about it.

You are told to determine j in Pj, aren't you? That suggests some kind of molecules, not atomic gas, isn't it?

As I wrote - information you have listed is wrong, as it doesn't yield reasonable answer. Please check if you have copied all numbers correctly.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Link The Worlds

  • New Member
  • **
  • Posts: 8
  • Mole Snacks: +0/-0
Re: Ideal Gas law problem; determining molecular formula
« Reply #4 on: September 04, 2010, 07:59:33 PM »
I solved for n by plugging in all the other variables, and got .013308933 moles. I don't
know what to do with this number to solve for the j in Pj. and I don't
see how there could only be .013308933 moles of P in 893 grams. I've been trying for hours to conceptualize this in my head, but to no avail.

I checked the paper and I copied all the numbers correctly.

Online Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Ideal Gas law problem; determining molecular formula
« Reply #5 on: September 05, 2010, 04:00:05 AM »
As I told you, there is something wrong with the data, so it won't let you calculate correct value of j.

Think: if you have 1 mole of ATOMS of P, how many moles of MOLECULES of P4 you will have?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links