Sapling #10

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Sapling #10

Postby cadytran1K » Sun Jan 17, 2021 10:07 pm

I'm having a hard time with the last part of this problem:

Calculate the equilibrium concentrations of N2O4 and NO2 after the extra 1.00 mol NO2 is added to 1.00 L of solution.

The Kc that I calculated is 11.2. I think my problem is solving for x. I set the equation up: [NO2]^2 / [N2O4] = ( 3.21 -2x)^2 / (0.435 + x) = 11.2 but can't seem to figure it out.

Giselle Granda 3F
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Re: Sapling #10

Postby Giselle Granda 3F » Sun Jan 17, 2021 10:14 pm

Since more product was added in the second part, equilibrium will shift to the left towards the reactants. Make sure that you add the extra mole/L to the 2.21M of NO2 to get your new initial concentration of NO2. Once you do this set your ice table in the same way you set up your last ice table. The only difference with this ice table is that your initial concentration for NO2 will be 3.21 and that instead of +2x for the change in concentration, it will be -2x because the reaction will now be proceeding in the reverse direction. This then means that N2O4 will have a +x change in concentration. Then all that is left is to set up your Kc equation the same way you normally would and solve for x using the quadratic equation. The value of x will be the one that does not give you negative concentrations when plugging them in to find NO2 and N2O4 equilibrium concentrations. Sorry if this is confusing but I hope it helps!

Lucy Wang 2J
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Re: Sapling #10

Postby Lucy Wang 2J » Sun Jan 17, 2021 10:17 pm

I think you need to factor the equation to be a quadratic so it will be Ax^2 + Bx + C = 0 and then use the quadratic formula x=[B +/- sqrt(B^2-4AC)]/2A. From this, you will get two answers and you need to eliminate the one that would give you a negative concentration value. Then you will have your x value and you can find [N2O4] and [NO2].

Ivan Chen 2H
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Re: Sapling #10

Postby Ivan Chen 2H » Sun Jan 17, 2021 10:17 pm

Looking at how you set the problem up, everything looks correct, assuming that the initial values are 2.21 mol/L NO2 and 0.435 mol/L N2O4. Have you tried finding Kc again with your new values and see if it equals 11.2 again? It might indicate something went wrong with the quadratic equation. You might have also squared the concentration of NO2 accidentally.

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Re: Sapling #10

Postby AlbertGu_2C » Sun Jan 17, 2021 10:20 pm

One thing that tripped me up when I tried to solve this question was how I had to add x to the initial concentrations in order to get the answer that you plug into the program. Hope this helps!

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