Posted by Cheyenne on Monday, December 20, 2010 at 4:47pm.
Calculate the number of moles of ALL gases present at that volume and pressure, and multiply it by 0.21 for the number of moles of O2. Then multiply that by Avogadro's number.
You could use the PV = nRT equation or the handy rule that one mole of gas occupies 2.4 liters at STP
4.0*10-4 m^3 is 0.40 liters
At STP, that would contain
0.4 l/22.4 l/mole = 0.179 moles
Apply a correction factor of 273/310 for the number density at the higher temperature, and multiply by .21 for the O2 mole fraction. 10^5 Pa is withing 1.3% of atmospheric pressure, so we can ignore the pressure correction factor 1.000/1.013
You should end up with about 0.033 moles of O2. Multiply by Avogadro's number to get the number of molecules.
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