Making X negligible

[Sean Phen]

What are the rules for when to make X negligible?

[Zoe Briquelet Miller 3A]

For equilibrium problems, X would be negligible if the Ka or Kb is 10^-4 or smaller because this would mean the reactants are heavily favored at equilibrium, and the change X would not be significant. However, it is important to check that using the shortcut was acceptable for the problem by finding the % ionization and making sure it is less than 5%.

[Kelly Tran 1J]

If K < 10^-4 , you can make the assumption that x is a really small value, which means you can use the shortcut of approximation. To be safe, you should confirm whether you can make the approximation by calculating the percent ionization. If (x/initial concentration) x 100 is less than 5%, your approximation is valid. However, if it is greater than 5%, your approximation is not valid, which means you would have to use the exact expression to solve for x.

[ellenulitsky Dis 1I]

It is the 5% rule, which is the same method as finding percent ionization! Hope this helps!

[Olivia Monroy 1A]

We just use the assumption that if K is less than 10^-4 the concentration is small enough to not affect the concentration of the initial species when the system has reached equilibrium. To test you can use the 5% rule (less than or 5%, finding % ionization). Makes solving for x easier/avoid using quadratic, but can solve with or without this assumption.

[Felicia Wei 1B]

x is negligible in the denominator of the equilibrium equation when the % ionization is less than 5%. First you can negate the x in the denominator and solve for [H+] or [OH-]. Then take that value to divide it by the initial and multiple by 100. If that number is less than 5% then your value can be accepted. If it is greater you need to go back and solve again without negating the x.

[Chloe Shamtob 2H]

You would make x negligible for equilibrium problems if the Ka or Kb is 10^-4 or smaller. Therefore, the change X would be too small to be significant. Make sure that the shortcut is allowed by finding the % ionization. If the percent ionization is less than 5%, your approximation is valid.

[Julianna Shimabukuro 1D]

As others stated, x is negligible if the Ka or Kb is less than 10^-4, and once you find x, if the concentration divided by the initial concentration is less than 5%.

[Stephanie Zhang 2K]

Hi!

Usually when the K value is less than 1E-4, that indicates that the reactants are more stable at equilibrium. So, that means that the change in the initial reactant concentration will not be very different from the equilibrium reactant concentration. Additionally, you should check afterwards that the "x" value you get is less than 5% of the initial reactant molar concentration, so that the change in the molar concentration is actually insignificant.

[Chinmayi Mutyala 3H]

When K is less 10^-4 is what Lavelle said in lecture but he also said to check the percent ionization so divide x by the initial concentration and if it's less than 5%, the assumption is okay.

[Sharon Kim 2A]

Usually it would be when the K value is 10^-3 but sometimes those values might be a little too high to the point reaching to 10^-2. So to be safe we start from considering X values that are 10^-4 and smaller to be insignificant.

[Crystal Pan 2G]

We make X negligible when X is 10^-4 or smaller. There is a guideline that says 10^-3, but if it were something like 9 x 10^-3, that would be closer to -2, which is why using 10^-4 is safer.

[Siwa Hwang 3G]

x is negligible when the equilibrium constant is less than 10^-4

[Ethan Goode 2H]

If k is less than 10^-4 then the X is negligible; however, do not make all X's equal to zero otherwise you will not be able to solve for X.

[Ivy Tan 1E]

Hi!
The value of X becomes negligible if the Ka, Kb, or Kc value is less than 10^-4. Hope this helps!!

[Talia Dini - 3I]

I believe that x would be negligible if it is less than 10^-4, however, just to be sure you can calculate the percent ionization. If the percent ionization is less than 5%, it would be fine to make the assumption that x is negligible.

[Eliana Carney 3E]

Hey Sean!

If the problem has a Kc value that is less than 1.0 x 10^-4, you can approximate that the value of X becomes negligible. You can do this because the small Kc value indicates that there is very little product formed at equilibrium, meaning that the concentration of the reactants isn't changed much.

One way to check that an approximation is okay is with the 5% rule. Basically, if the value of X is less than 5% of the initial concentration, the approximation was okay. If not, then you cannot approximate for the given question. Hope this helps!

[Ziyan Peng 3A]

first you typically look at if K ≤ 10^-4, and if the question satisfies that you go ahead with neglecting x in the denominator, but you double check afterwards to see if x value < 5% of the initial concentration.

[Halle Villalobos 3E]

Hi! When K < 10^-4, the change in X will be so small that we can make it negligible. While the book considers X negligible when K < 10^-3, Lavelle notes that K < 10-4 is a better rule to work from. I hope this helps!

[Neel Sharma 3F]

We can consider x negligible when the equilibrium constant, K, is smaller than 10^-4. A more precise and specific way is the 5 percent rule. If x is more than 5 percent of the initial concentration, it can not be considered negligible. Hope this helps!

[Justin Zhang_1A]

You can ignore X when the K constant is less than 10^-4.

[Evelyn Silva 3J]

If the equilibrium constant is less than 10^-4 you can assume the change is neglible. However, if you have a value that's really close to 10^-4 or exactly that value then I like to calculate the percent ionization just to make sure the assumption is correct. If you get something greater than 5% you wouldn't be able to ignore x.

[VSU_3F]

If the K value is less than 10^-4, X is negligible.

If less than 5% of the acid is deprotonated when you ignore X, then X is also negligible.

[Melanie Lin 3E]

Lavelle mentioned that Ka or Kb should be less than 10^-4 for x to be negligible. I actually had a previous teacher say that K needed to be less than 10^-3 of the original concentration but it's definitely best to listen to Lavelle. Hope this helps!

[Kelly Ha 1K]

The change in X is negligible when K is < 10^-4. You can check if X was supposed to be considered negligible by using the 5% rule (if percent dissociation is less than 5% the approximation was valid).

[Katie Phan 1K]

I prefer just plugging the value I got for x / initial concentration and see if it's above 5%. If it is, the value of x is not negligible and you have to solve the quadratic equation likely. If it's under 5% you're good.

[Lauren Mungo 1K]

If K is less than 10^-4 then you can consider it negligible, or if % ionization is less than 5%.

[Dane_Beasley_1E]

If the K value is less than 10^-4 and/or the % ionization is 5% or lower than it would make x negligible.

[AndrewNguyen_2H]

Typically, when K is very small or large and you aim to subtract x from the side in which the reaction is favored. Using the 5% rule is a way to check if your assumption was correct.

[Geethika Janga 1L]

If the value of Ka/Kb is very small, typically less than 10^-4, then you can consider the X value to be negligible when solving your equation. However, always remember to check with the 5% rule after you find the value of X, this means that you should double check whether your calculated value of X is less than 5% of your initial concentration.

[Ethan Laureano 3H]

When the equilibrium constant is smaller than 10^-4, then you can try. Of course, after trying, check to see if the x value is less than 5% of the initial concentration.

[Bhuvan Kommineni 3L]

Don't think of this as a math rule... think of it conceptually. Under what conditions can X be emitted? This would be when the K value is extremely small, that it doesn't make a difference.

[TiffanyBrownfield 2I]

I believe Lavelle said that if the K is less than 10^-4, then you can approximate. However, if you are not sure you can check by seeing if your final concentration is less than 5% of the initial concentration. If this is the case (<5%), then you can disregard having to use the quadratic formula.

[MariaCassol1L]

At first, you should use the Ka/Kb value to see if you can approximate if the equilibrium constant is below 10^-4 then we say that x is so small that the impact it will have on the concentration of the reactant is almost zero, so we don't consider it. However, after you finish your calculation you should check %deprotonation/%protonation and if it is more than 5% you should repeat the calculations including the x.

[Kathy_Li_1H]

Hi! X is neglible when the initial concentration for the reactant is 1000x larger than the Kc value.