Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Syringe: probably a really simple mistake that I can't seem to catch!

  1. Dec 1, 2008 #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
    KE=1/2 m v^2=3/2kT

    3. The attempt at a solution
    Hi ok so I'm new here! Sorry if I'm doing anything incorrectly. I've been working on this problem for quite some time and I have a feeling that the answer is staring at me in the face, but I can't seem to catch my mistake. I'm literally solving for the variable v. I'm so close! Please help me!

    1) Find the total # of gas molecules: 6.5*10^19 (100/20)= 3.25*10^20
    2) Solve for temperature: PV=nkT: (100,000) (0.02)= (3.25*10^20)* (1.38*10^-23)T
    3) 32g/mol * 6.022*10^23=1.92704*10^25grams=mass of O2
    4) Solve for Vrms: .5(1.92704*10^25)v^2= 3/2(1.38*10^-23)(445.93088)
    Vrms= 3.09519902*10^-23


  2. jcsd
  3. Dec 1, 2008 #2
    Consider equation 3) again. It says that there are 1.93*1025 grams of O2 in the syringe. That is one heavy syringe! The mass of O2, in equation 4), is actually the mass of one oxygen molecule. How do you find that?

    You made a mistake in calculating the mass of oxygen in equation 3). 32g/mol means that one mole of oxygen (or NA molecules) weighs 32g. To find the mass of the molecules, you need to multiply the no. of moles of oxygen with the molar mass.

    PS: Also, if you are using the total mass of the molecules in eqn 4), the KE formula needs to be modified to KE = 3/2 NkT = 1/2 m<vrms>2, where N is the no. of molecules.
    Last edited: Dec 1, 2008
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook