March 28, 2024, 11:27:13 AM
Forum Rules: Read This Before Posting


Topic: calculating grams given pressure, volume, and temperature  (Read 32328 times)

0 Members and 1 Guest are viewing this topic.

Offline laxplayer

  • Regular Member
  • ***
  • Posts: 45
  • Mole Snacks: +2/-1
  • Gender: Female
calculating grams given pressure, volume, and temperature
« on: November 05, 2007, 07:10:00 PM »
The problem states:
Calcium hydride, CaH2, reacts with water to form hydrogen gas.

    CaH2(s) + 2H2O(l) Ca(OH)2(aq) + 2H2(g)

This reaction is sometimes used to inflate life rafts, weather balloons, and the like, where a simple, compact means of generating H2 is desired. How many grams of CaH2 are needed to generate 18.0 L of H2 gas if the pressure of H2 is 770 torr at 17°C?

I have no idea how to even start this problem, so could someone point me in the right direction :]

Offline Sev

  • Full Member
  • ****
  • Posts: 231
  • Mole Snacks: +43/-6
  • Gender: Male
Re: calculating grams given pressure, volume, and temperature
« Reply #1 on: November 05, 2007, 07:32:48 PM »
Just use PV=nRT to find mol of H2.  Work backwards for mol of CaH2.

Offline laxplayer

  • Regular Member
  • ***
  • Posts: 45
  • Mole Snacks: +2/-1
  • Gender: Female
Re: calculating grams given pressure, volume, and temperature
« Reply #2 on: November 05, 2007, 08:07:26 PM »
So there are .766 mol of H2 and .383 mol of CaH2?

For H2 I did (18)(1.0132)/(.0821)(290.15)

For CaH2 I divided .766 by two because of the mol ratio, I think that's what you're supposed to do..

Offline constant thinker

  • mad scientist
  • Sr. Member
  • *****
  • Posts: 1275
  • Mole Snacks: +85/-45
  • Gender: Male
Re: calculating grams given pressure, volume, and temperature
« Reply #3 on: November 05, 2007, 08:07:54 PM »
To expand on Sev's post...

PV=nRT is the ideal gas law. It's extremely useful in gas stoichiometry.
P= Pressure
V= Volume
n= moles (sometimes substitute with "m/M" m=mass of sample M=molecular weight)
R= a gas constant, and it depends on the units you are using. You will need to look this up, I'm sure wikipedia has them.
T= temperature in Kelvin (always Kelvin for gas equation and most mathematical equations because it does not have negatives, and you'll never hit 0 in a real world problem)

[edit] You already answered before I got my post off.

Your answer is right. I don't know where you pulled your numbers from to get .766 mols H2, but you came out with the right number of mols for CaH2
"The nine most terrifying words in the English language are, 'I'm from the government and I'm here to help.' " -Ronald Reagan

"I'm for anything that gets you through the night, be it prayer, tranquilizers, or a bottle of Jack Daniels." -Frank Sinatra

Offline laxplayer

  • Regular Member
  • ***
  • Posts: 45
  • Mole Snacks: +2/-1
  • Gender: Female
Re: calculating grams given pressure, volume, and temperature
« Reply #4 on: November 05, 2007, 08:13:02 PM »
do you know if what I did is correct though?

Offline constant thinker

  • mad scientist
  • Sr. Member
  • *****
  • Posts: 1275
  • Mole Snacks: +85/-45
  • Gender: Male
Re: calculating grams given pressure, volume, and temperature
« Reply #5 on: November 05, 2007, 08:19:19 PM »
Here is how I did it...

PV=nRT solve for n so you can find the mols of H2
n= PV/RT
n=(770 torr * 18 L)/(62.364 * 290°K)
n=.766
.766 mol H2

Now multiple by the ratio for calcium hydride to hydrogen to get mols of calcium hydride.
.766 mol H2*(1 mol CaH2/2mol H2)
.383mol CaH2
"The nine most terrifying words in the English language are, 'I'm from the government and I'm here to help.' " -Ronald Reagan

"I'm for anything that gets you through the night, be it prayer, tranquilizers, or a bottle of Jack Daniels." -Frank Sinatra

Offline laxplayer

  • Regular Member
  • ***
  • Posts: 45
  • Mole Snacks: +2/-1
  • Gender: Female
Re: calculating grams given pressure, volume, and temperature
« Reply #6 on: November 05, 2007, 08:23:59 PM »
oh, I did (1.0132 atm * 18 L)/(.0821 * 290 k)
Where did you get 62.364 from?

Offline constant thinker

  • mad scientist
  • Sr. Member
  • *****
  • Posts: 1275
  • Mole Snacks: +85/-45
  • Gender: Male
Re: calculating grams given pressure, volume, and temperature
« Reply #7 on: November 05, 2007, 08:28:14 PM »
That is the R value if you were to use torr. You did it right though, I see what you did. You converted the torr to atm.
"The nine most terrifying words in the English language are, 'I'm from the government and I'm here to help.' " -Ronald Reagan

"I'm for anything that gets you through the night, be it prayer, tranquilizers, or a bottle of Jack Daniels." -Frank Sinatra

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: calculating grams given pressure, volume, and temperature
« Reply #8 on: November 06, 2007, 03:07:02 AM »
0.08205783 L*atm/(K*mol)
8,314472 L*kPa/(K*mol)
8.314472 J/(mol*K)
62,3637 L*mmHg/(K*mol)
83,14472 L*mbar/(K*mol)
1.987216 cal/(K*mol)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links