What is defined as negligible?

[Lucas Lopez]

On the Achieve HW, I had an interesting situation occur, I solved a problem where X should have been negligible, and was prompted that I had made a mistake. The problem set the Kc as a number in the 10*-5 range, and it was asking for the percentage protonated. In the end, the percent protonated was over 5%, and thus x could not be negligible. How could I know this before beginning my problem? As to not get it wrong next time.

[Dashrit_Pandher_1J]

When K > 10^3 or K < 10^-3 we do typically say X would be negligible, and the way to check is through the percent protonated. I don't believe there to be another way to check besides through doing percent protonated. For Achieve it might be a good rule of thumb to always check that your X truly is negligible by also calculating the percent protonated. Professor Lavelle did mention that he would very rarely give us a problem where X could be considered negligible and approximation is wrong. If he did give a problem like that then he would explicitly ask us to solve for the percent protonated.

[Joan Thyagarajan 2G]

I believe that the k value is defined as negligible when it is greater than 0.001 or greater than 1000 because then, what ever is left of the minority side of the reaction-product if it is 0.001 and reactant if it is 1000-is not going to affect the calculation of Q because it is such a small value that is being used in an expression that has much bigger numbers

[bella moore]

You know that the change in x is negligible if K is extremely small (10^-4 or smaller), then you can assume that, in the difference in the initial - x, x is negligible. You can make sure this assumptions correct by making sure the percent pronated is less than 5%

[emmavandal1K]

X can be considered negligible when K is very small (<10^-3). Since your K was even smaller than that, it would make sense that approximation should work in this instance. It may just be one of the situations in which approximation does not work, and I am not sure if there is a way to check beforehand. I believe you have to go through with the calculation of percent protonated to determine is approximation is valid.

[chloeremigio1e]

Like others have said, x is negligible when it is very small (k<10^-3). I ran into a similar problem while working out one of the homework problems as well, but I think the only way to verify that x is negligible is by double checking that the percent protonated is less than 5%. When I looked at the worked out solution that achieve gave, they took steps to double check that x was negligible and found out that it wasn't, so I'm not sure if there's a way to know beforehand.

[NISHA PATEL 1C]

In lecture Professor Lavelle defined that X is negligible when K < 10^3-. I also ran into the same problem when working on that question. You can double check your work by finding the percent protonation. If this percentage is above 5%, you cannot neglect X.

[Jennifer Jin]

When the x is 1000 less than the initial molarity (M).

[205750231]

I also experienced this problem. If the percent protonation is greater than 5%, then X cannot be considered negligible. This is found by dividing the equilibrium concentration of the final protonated substance by the initial concentration of the substance being protonated.

[Miles Dailey 1K]

If you get a percent protonation above 5%, you cannot assume that X is negligible, and will have to redo calculations with X included. This means you will likely have to use the quadratic formula in your calculations.