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Homework Help: Cant determine vrms, even though equations are all correct. . please help!

  1. Apr 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A syringe of volume 20 ml has just been used and now lies empty on the doctor's desk. The pressure in the office is 100,000 Pa. Assume that the air is an ideal gas consisting of nitrogen N2 (80%) and oxygen O2 (20%) molecules. k=1.38×10-23 J/K and the molar mass of oxygen is 32 g/mol.

    The syringe must be heated to high temperatures to sterilize it. When the syringe is at its maximum temperature, the number of oxygen molecules is 6.5 x 1019 (but the pressure, volume, and percent oxygen remain the same), what is the rms speed of the oxygen molecules?

    2. Relevant equations
    vrms = root 3kT/m

    3. The attempt at a solution
    i know its the equation above that gets me vrms, so i just need T and m
    T is 445.9K (using T= PV/Nk)
    and m is 5.31e-23g (from m = 32g/6.022e23)

    so using vrms = root3kT/m i get 18.645m/s

    but this is wrong for some reason.. plaease help thanks!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 5, 2009 #2
    For the value of Boltzmann's constant given, the mass needs to be in kilograms. That way the units on either side will be equal.
  4. Apr 6, 2009 #3
    Thank you. That solved the problem. UNITS!
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