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Posted by on Sunday, July 15, 2012 at 12:07am.

The reaction for my lab is
Mg(s)+2HCl(aq)--> MgCl2(aq)+H2(g)

I need to find the pressure of the hydrogen gas.
Initial Pressure = 0.98692 atm
Total Pressure = 1.64816 atm

I don't know if I calculated the pressure of hydrogen gas correctly. I used the formula Total Pressure=Initial Pressure+Pressure of H2+Pressure of H2O
This is what I got for an answer.
Pressure of H2=1.64816-0.98692-0.0313=0.6926atm

  • Chemistry - , Sunday, July 15, 2012 at 12:25am

    Where did the 1.64816 come from?
    Usually these are done by collecting H2 gas over water. Then the room pressure, which I assume is the 0.987 atm - vapor pressure of water at room T is pressure of dry H2 gas at those conditions. This assumes the pressure inside the collection vessel and outside the vessel were the same.

  • Chemistry - , Sunday, July 15, 2012 at 12:34am

    In the lab I had a strip of magnesium that weighed 0.029 grams and I added 15 milliliters of 0.5 M hydrochloric acid into a flask and I never used any water during the lab. So I don't really understand where I would get the water vapor pressure.

  • Chemistry - , Sunday, July 15, 2012 at 12:53am

    OK. I don't know what you did. You must have generated the H2 gas and measured the total pressure at the end. So total P - pH2O - initial P would be P of dry H2. If so what you have looks ok to me. The vapor pressure of the water must be the 0.0313 (that's equivalent to 23.7 mm Hg which is about right for a temperature of 25C.

  • Chemistry - , Sunday, July 15, 2012 at 12:54am

    The pressure of the water vapor comes from the water in the HCl(aqueous).

  • Chemistry - , Sunday, July 15, 2012 at 12:58am

    Oh ok and I used a device called a Xplorer GLX and it measured the pressure of the flask before I put in the magnesium and hydrochloric acid and then it measured the pressure once the reaction was complete. But thank you for all of your help!

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