# AP Chemistry

posted by Frank

Given: N2O4 (g) « 2NO2 (g) @ 25 degrees celcius, Kc is 5.84 x 10^-3.

(A) Calculate the equilibrium concentrations of both gases when 4.00 grams of N2O4 is placed in a 2.00 L flask at 25 degrees celcius.

(B) What will be the new equilibrium concentrations if the volume of the system is suddenly increased to 3.00 L at 25 degrees celcius.

Would you be able to find the moles of N2O4 by taking 4.0g/92.02 g/mol. Then in order to find mol NO2 can you just multiple mol N2O4 by 2 because the equation is 2:1. Then divide both by by liters to get their concentrations.

Or do you have to Calculate the number of moles of N2O4 and hence the concentration (C) in moles/L. Then say that at equilibrium let the concentration of NO2=x so Kc=[NO2]^2/[N2O4] Kc=(x)^2/(C-x)= 5.84x10^-3 we know C so solve for x

If it is the second way can you please guide me on how to solve for X

1. DrBob222

It is the second way. And one problem I am having (I looked at your post last night but didn't post a response) is I can't really tell which way the reaction is going since the arrows seem to be backwards to me. If I write an equation I usually write it forward. Of course I understand that equilibrium reactions can go either way; it's just that this is a little unusual for me. Can you clarify that for me; i.e., is the reaction
2NO2 ==> N2O4 with Kc = 5.83 x 10^-3 or is it N2O4 ==> 2NO2 with Kc = 5.83 x 10^-3?

2. Frank

It is N2O4 ==> 2NO2 with Kc= 5.83 x 10^-3 (the arrow is actually a double headed arrow to symbolize equilibrium) Sorry for the misunderstanding! I didn't realize the arrows were like that

3. DrBob222

Thank you for clarifying.
We can't control spacing on the board. You can write it as you usually do but I shall write mine in a column instead of a row.
initial:
N2O4 = 4.00/92.01 = moles and that divided by 2L = 0.02174M
NO2 = 0

change:
N2O4 = -xM
NO2 = +2xM

equilibrium:
N2O4 = 0.02174-x M
NO2 = 2x M

Set up (2x)^2/(0.02174-x) = 5.84*10^-3 and solve for x. Don't forget that NO2 is 2x. I usually check myself when I'm finished by substituting (NO2) and (N2O4) into Kc expression and see if I get Kc. Tells me if my math is ok. I think this one is. If you want to check yourself, I found x = 0.00495 to make (NO2) = 2x that and (N2O4) = 0.02174-0.00495. I hope this helps.

4. Frank

Thank you!
For part B I did the same thing only this time I used 3.00 L. However I can't seem to find the value of x... I am coming up with 0.0191, but when I substitue that in I do not come up with 5.84 x 10^ -3

5. Frank

Given: N2O4 (g) « 2NO2 (g) @ 25 degrees celcius, Kc is 5.84 x 10^-3.

(A) Calculate the equilibrium concentrations of both gases when 4.00 grams of N2O4 is placed in a 2.00 L flask at 25 degrees celcius.

(B) What will be the new equilibrium concentrations if the volume of the system is suddenly increased to 3.00 L at 25 degrees celcius.

For part B I did the same thing as part (a) only this time I used 3.00 L. However I can't seem to find the value of x... I am coming up with 0.0191, but when I substitue that in I do not come up with 5.84 x 10^ -3

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