Rounding E Values in ICE charts

[Victoria Dao 3G]

Hi! I am still a little confused as to when we are allowed to round the E value in the ICE chart. What classifies a K value as being small enough that it won't affect the equilibrium concentration?

[Ryan Laureano 3I]

If the K value is smaller than 1x10^-3 than you can ignore its effect when subtracting k from a concentration.

[Megan Chan 3A]

Adding on to the previous reply, if you approximate by omitting x, you should also check that it is less than 5% of the initial concentration to make sure it is okay to approximate. Otherwise, the quadratic formula should be used to solve the equation.

[MichaelRaad_1F]

I think if k is smaller than 10^-5 then we can assume it is 0 when calculating equilibrium.

[Darlene Lien 3E]

Hi, I believe Dr. Lavelle mentioned in the lectures that if K<10^-4 then the change in x is very small where it essentially becomes negligible, but I believe it could sometimes work when K<10^-3. Like the comment above said, a good rule of thumb is to use the 5% rule to check if the estimation works!

[derickngo3d]

If the K value is less than 1.0 x 10^-3, then the x can be dropped.

[Josh Chou 3K]

I think that if k < 10^-3 (pretty sure Dr. Lavelle said k < 10^-4 if you want to really be safe) and if the starting concentration(s) are more than 1000 times greater than the k value, you can estimate in the E row by omitting the +x or -x.

[Eliana Carney 3E]

Hey Victoria!
Basically, we are allowed to round the equilibrium value in an ICE chart when Kc < 1.0x10^-3.
One way to double check if rounding the equilibrium value in the ICE chart was appropriate is to make sure that the X value you calculated using the rounding is less than 5% of the initial concentration. If it is, the approximation was okay and if not you will have to do the full calculation for X without rounding.
Also, if you're not sure wether or not you're allowed to round, I usually don't round and just do the full calculation. It may not be very time efficient, but better safe than sorry. Hope this helps!

[Kelly Ha 1K]

If the K value is less than 1.0 x 10^-4 then the change in x (-x or +x) can be omitted. (I usually use 10^-4 rather than 10^-3 as the cutoff to be safe). Also, as stated in a previous post sometimes it's safer to just do the whole calculation without worrying about whether the approximation is valid just to make sure there are no errors, but this can depend on whether or not you're pressed for time.

[Gustavo_Chavez_1K]

I remember in lecture Lavelle saying that if the k value is 10^-4 or smaller then x can be omitted in the E section of the ICE table.

[Kiana Tashakori 1D]

You can leave out x if the k value is equal to or less than 10^-4, because at that point k is so small that it wont really affect the values.

[Arnav Saud 2C]

If the K value is less than 1E-3, you can set the x values from the ICE tables to 0.

[Brandon Gruender 3F]

Normally, anything smaller than 1x10^-4 is safe to assume, however I would always check that it is less than 5% of the initial concentration. If it is not then you need to use the quadratic equation to find x for that problem.

[Colin Squire 3B]

When the equilibrium constant is less than 1x10^(-3), we can assume that they x value is negligible. Dr. Lavelle also showed us in lecture that if we calculate the percent ionization and it turns out to be less than or equal to 5%, we can use this "short cut."

[305405193]

is this why he referenced it to money when he stated if someone has 10,000 dollars and gives 10 dollars to someone, they basically have 10,000 dollars? Is that what he signified, by basically rounding if the E is less than a certain value?

[Annette Fishman]

If the K value is less than 1.0 x 10^-3, then the x can be dropped. Lavelle prefers we drop it at 1.0 x 10^-4 to be more safe, but essentially its when the K value is so small it doesn't affect the overall concentration.

[DMaya_2G]

The x can be dropped if the K value is less than 1.0 x 10^-3.

[Nick Saeedi 1I]

If k is less than 10^-4 we can assume that x is very small so then we can round. Also, if the assumption is made, the percent ionization has to be less than 5 percent.