How would i calculate the percentage of oxygen gas in this mixture?

A gaseous mixture of oxygen gas and Kr has a density of 1.104 g/L at 435 torr and 27 degrees Celsius.

calculate the percentage of oxygen gas in this mixtureHow would i calculate the percentage of oxygen gas in this mixture?
Step One

Find the volume of Gas at STP

P = 1 atmosphere

T = 273 oK

V = ???



P1 = 435 T * 1 atmosphere / 760 T = 0.5724 atmospheres

T1 = 27 + 273 = 300 oK

V = 1 L I've chosen 1 Liter because I know how much 1 liter's mass is (ie 1.104 grams).



PV/T = P1 * V1 / T1



1 * V / 273 = 0.5724 * 1 / 300

V = 0.521 L



Step Two

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Find the number of moles of gas in the mixture.



1 mole of a gas at STP occupies 22.4 L

x mole this gas occupies 0.521 L



22.4 x = 0.521

x = 0.0233 moles.



Step Three

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Find the number of moles of oxygen and Krypton in this mixture.



Let the number of moles of oxygen = x

Let the number of moles of Kr = y



x + y = 0.0233

32x + 84 y = 1.104



the moles add up to 0.0233 moles

The mass is the molar mass of oxygen (32) * moles + molar mass of Krypton (periodic table 83)



Solving for x and y we get

x = 0.0164 moles oxygen

y = 0.00689 moles Kr



Step Four

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Find the percent oxygen. I'm going to use moles as my base and not grams.



The total number of moles = 0.0233 moles.



0.0164 / 0.0233 * 100 = 70.4 %



Comment

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The percentage by mass is quite different. That comes out to

0.525 / 1.104 * 100 = 52.5 %



Where did I get 0.525 You might ask? The number of moles * molecular mass of oxygen =

0.0164 * 32 = 0.5248 grams.