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AP Chemistry Exam Prep

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You have a closed container filled with 1L of pure air at STP. Assume that this air is made of only nitrogen (79%) and oxygen (21%). What are the partial pressures of nitrogen and oxygen in the box?Hint: XN = PN/PTotal= NN/NTotal
  1. PN = 0.5 atm PO = 0.5 atm
  2. PN = 1.0 atm PO = 1.0 atm
  3. PN = .21 atm PO =.79 atm
  4. PN = .79 atm PO = .21 atm
  5. PN = .75 atm PO = .25 atm
Explanation
Answer: D. Solved using partial pressure equations.

XN=mole fraction of nitrogen
NN=moles of nitrogen
NTotal=moles total
X0=mole fraction of oxygen
No=moles of oxygen

Since 79% of the air is nitrogren, the mole fraction (xN) is .79. (XN=NN/NTotal). Likewise, the mole fraction for oxygen is .21. This is calculated by realizing that 79% of the air is nitrogen, therefore 79% of the molecules (or moles) are nitrogen. So, you don’t need to know NN or NTotal to calculate this.

Partial pressure equations are as follows:

XN = PN/PTotal= NN/NTotal
.79 = PN/PTotal
.79*PTotal = PN
.79 * 1.0 atm = PN = .79 atm

P0 = PTotal - PN = 1.0 atm - .79 atm = .21 atm
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