[jmg12]

Hi I just did a lab in which we determined the volume of oxygen gas. We used a gas collecting apparatus. We had a mixture of KClO3 - KCl with MnO2(catalyst) and heated it up to extract the O2. The O2 pushed water from a flask through a tube into a beaker. I have three questions about this technique.

1) Why must the apparatus cool to room temp before the syphon between the flask and beaker is disconnected?

2) What is more accurate, calculating the moles of oxygen from the volume formed or the weight of oxygen generated?

3) If you failed to subtract the water vapor pressure from the total pressure, would your molar volume be too high, the same, or too low? Why?

Thank you so much. Any help would be appreciated.

[opti384]

I'll just give you my thoughts. Any other ideas are of course welcome and please tell me if I got something wrong or if you think otherwise.

2) I think the latter will be more accurate because if you were to calculate the moles (assuming that you use the ideal gas equation), you need the pressure, temperature, and the volume. Therefore, there is a big chance of getting erroneous values as you measure all the three.

3) PV = nRT

V/n= RT/P

If you fail to subtract the water vapor pressure, the pressure of oxygen will be greater than its real value. Therefore, the molar volume will be lower than it should be.

[jmg12]

I completely agree with your answer to the third question and understand your logic to the second. But do you think sig figs have anything to do with that question? I'm not sure and I'm hesitant on adding that in my answer. Also, for the first question, if the test tube that is being heated (thus releasing the oxygen gas) isn't allowed to cool, would the temperature of the gas just be hot? Or would that affect something else?

[opti384]

But do you think sig figs have anything to do with that question?

Could you elaborate more about this? So what's your idea about sig figs?

if the test tube that is being heated (thus releasing the oxygen gas) isn't allowed to cool, would the temperature of the gas just be hot? Or would that affect something else?

I didn't get this one. So are you saying that would the heating the test tube affect the temperature of the oxygen?

[jmg12]

This is the exact experiment we did.
You'll probably need to get an understanding of how it was set up for that first question I asked. We heated the test tube, then allowed for it to cool down to room temp before making measurements of the volume of water (amount the O2 pushed into the beaker).
http://wwwchem.csustan.edu/chem1102/molarvol.htm

When I measured the volume, I used 3 sig figs because I used a graduated cylinder. When I calculated the weight, I only could use 2 sig figs. So many that extra sig fig makes using the volume more accurate.

[opti384]

We heated the test tube, then allowed for it to cool down to room temp before making measurements of the volume of water (amount the O2 pushed into the beaker)

So your question is why wait for the test tube to cool right? That's probably because you want to use the volume of oxygen of the room temperature.

When I measured the volume, I used 3 sig figs because I used a graduated cylinder. When I calculated the weight, I only could use 2 sig figs. So many that extra sig fig makes using the volume more accurate.

If that is the case, it also makes sense.

[jmg12]

Thank you opti, I appreciate all of your time and help.