Chemistry page 91 and 92 question 6

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sarita0518
Posts: 143
Joined: Fri Feb 22, 2019 3:08 pm

Chemistry page 91 and 92 question 6

Post by sarita0518 » Fri Mar 15, 2019 11:56 pm

hi-

can you please explain what does it mean by mole fraction. And why do we not multiple the value (.6/1.32) by 2 since the element has a coefficient of 2 when calculating the mole fraction?

Thanks so much!
NS_Tutor_Will
Posts: 685
Joined: Fri May 25, 2018 9:15 am

Re: Chemistry page 91 and 92 question 6

Post by NS_Tutor_Will » Sun Mar 17, 2019 8:52 am

Mole fraction is just the fraction of the total moles of a sample that is made up by a particular molecule. In other words, it is the amount of the species of interest over the total amount:

Xa = na / na + nb + nc + ...

In this question, we have to use partial pressure to find "pressure fraction" (which, in gases, will be equivalent to mole fraction).

Kp (equilibrium expression) = PA(6) / PA(3)^2 = 2

PA(6) partial pressure is 0.72, so we can plug that in.

PA(3)^2 = 0.72 / 2 = 0.36 atm

Then, we take the square root to find PA(3) = 0.6

Now, to solve for the fraction:

0.6 / 0.6 + 0.72 = 0.6 / 1.32, which is slightly less than 1/2.

Thus, the mole fraction should be slightly smaller than 0.5.
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