1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Into to Ideal Gas Law Problem

  1. Jan 13, 2012 #1
    1. The problem statement, all variables and given/known data

    A vacuum pump on Earth can produce a vacuum with a pressure as low as 1.00*10^-8 torr. How many molecules are present in each mL at a temperature of 27.0 degrees Celsius?

    V = 1mL = 10^-3 L <-- How many sig figs should I use here? Should I make it 1.00*10^-3L or keep it 10^-3L?
    P = 1.315789 atm
    T = 300.15 K
    n = ?
    2. Relevant equations

    Pv = nRT

    3. The attempt at a solution

    PV =nRT

    n = PV / RT

    = (1.315789atm)(1.00*10^-3L) / (0.08206 Latm / Kmol)(300.15K)

    = 5.342156*10^-16mol * 6.022*10^23 molecules

    = 3.22*10^8 molecules / L <--- How do I change my answer back to how many molecules/mL and how many sig figs should my answer reflect? 1 or 3?
  2. jcsd
  3. Jan 13, 2012 #2


    Staff: Mentor

    isnt a liter = 1000 mL
  4. Jan 13, 2012 #3
    Yeah so would I just multiply my answer by 1000?
  5. Jan 13, 2012 #4


    Staff: Mentor

    okay so think a liter is 1000 times bigger than a mL right? so that means it can hold 1000
    times the # of molecules that an mL would have? so do you multiply or divide?
  6. Jan 13, 2012 #5
    my mistake, yeah I'd definitely divide my answer by 1000. Also for sig figs, since the question says "one milliliter" how many digits should I interpret that as? Should I let the volume = 1mL, which would give me one sig fig in my answer or should I just make it 1.00mL and include 3 sig figs in my answer?
  7. Jan 13, 2012 #6


    Staff: Mentor

  8. Jan 13, 2012 #7


    User Avatar

    Staff: Mentor

    Your answer was already correct - you used 10-3L as a volume, so you calculated number of moles in 1 mL. You added L at the very end of the calculation out of nothing, confusing yourself.

    Question says "in each mL" which probably means "exactly mL" - so there is as many significant digits as you want. Sure, it doesn't make sense to report all digits your calculator displays - I would go with three.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook