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Homework Help: Confusing gases, help with lab report needed

  1. Apr 13, 2004 #1
    Hi all! This what I've done: First I pumped underpressure into a glass container, checked the manometer's reading and then weighed the container on two different scales. Then I opened the valve a bit, checked pressure and weighed the bottle etc. I did this 10 times. So what I've got is 10 pressure-mass readings.

    Now the report part. I first had to draw a measured points into (p,m)-coordinate grid. Then I did linear regression and derived a formula:


    so m is a function of p. With the slope (VM/RT) I could determine the mass of the air inside the bottle in room temperature and in NTP. No problems here. Next, they want me to determine the density of air with the help of VM/RT and the mass of air. It would be a fairly easy calculation if I knew the volume of the container, but I don't. Also, the molar mass of humid air (humidity 40%) is unknown.

    I'm completely stuck on this one. A prod into the right direction would be greatly appreciated!
  2. jcsd
  3. Apr 14, 2004 #2
    Well, I doubt I will be very helpful, but no one else is helping you yet.

    Can you measure the container and make a guess as to the volume?

    This may or may not be helpful: Density of Dry air at 10 degrees C is 0.00125 g per cc.
    At 30 C, it's 0.00116.

    Since you found out the mass and the density can be found depending on the temp, you can get the volume? Take away the weight of the container? And remembering that the more humid the air, the less weight, right? Because the tiny water molecules weigh less than dry air. Something like that.

    Good luck.
  4. Apr 16, 2004 #3
    Thanks for your input. The values you gave seem to agree with those I got. It seems I did a pretty stupid mistake, the volume WAS known - I just hadn't noticed it. It was written on the container. Well, the rest of the report wasn't that hard, and I've got it done now.

    God, I feel stupid.. :frown:
  5. Apr 16, 2004 #4
    Don't beat yourself up for not noticing something. Now you will probably always be very observant, so it was a good lesson to learn, right? I know one thing; I look at mechanical things and tools very closely, and doing so enables me to understand what they are used for and how to handle them. I look over my physics labs stuff very, very carefully before I go ahead with the experiment, and so far that's been very helpful.

    Good luck for the rest of the semester!!!
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