# V of gas from decomposition

1. Oct 3, 2005

### Pengwuino

So heres the problem I am faced with... dum dum dummmm

So first I figured that there will be 30.6g of hydrogen peroxide. Then I figured that there are 0.4498 moles of O2. Then using the ideal gas law...

V=nRT/P

I got ((0.4498)(0.08206)(27+273.15))/(746/760) = 11.3 Liters of O2

But supposedly im wrong. Where did I go so horribly horribly wrong?

Oops, and i just punched in 22.6 Liters and it says I'm right.... so where did I divide by 2 where I shouldn't have?

Last edited: Oct 3, 2005
2. Oct 3, 2005

### Tom Mattson

Staff Emeritus
It's impossible to locate your error since you have not shown your work.

3. Oct 3, 2005

### Bystander

You didn't read/remember the stoichiometry of the decomposition.
22.6? Gotta be a TA solving the problem sets --- and getting wrong answers, as is usual. You've been told where you didn't divide by two when you should have, and it's probably the same place the TA multiplied mistakenly.

4. Oct 3, 2005

### Pengwuino

Everything was already given, all I had to do was punch in the numbers. All i needed was moles and the temperature conversion and both were done correctly. The R constant is correct as well.

5. Oct 3, 2005

### Pengwuino

So 11.3 was correct?

I've been noticing a few of these problems are absolutely wrong lately in the homework. One problem had a very simple PV=nRT problem, EVERYTHING except 1 variable was missing and I did it and couldn't figure out the right answer. I show it to 4 other people... one about to graduate with his bs in physics, one his masters in physics, and 2 other people and no one could figure out what was wrong. The answer made sense (small volume, low pressure, normal temperature meant even smaller moles value) and it wasn't acording to the homework program.

6. Oct 3, 2005

### Bystander

Mole of peroxide yields half mole of O2. It's called a sitting duck.

Collateral duties of TAs include solving problem sets, checking answer keys, and keeping keys up to date when instructors change the numbers from year to year to defeat the frat-rat files --- these duties are usually performed in lackluster fashion, if at all.

Your job as a student is to bring necessary corrections to TAs' and instructors' attentions in as diplomatic a fashion as possible --- if they don't take it gracefully, you've got end of term evaluation forms.

7. Oct 3, 2005

### Pengwuino

Well was I right with the first calculation (11.3)?? Im getting a little confused by everyone here :P

8. Oct 3, 2005

### Bystander

Half mole, 11 liters, yes.