How small of a K number do you approximate?

[Ashley Johnson 2G]

In the lecture from 1/12, in the problem where we were calculating the pH of the weak acid CH3COOH, Dr. Lavelle explained that Ka is small so we approximated the X value out of the denominator when solving x^2/0.10-x=1/8x10^-5
At what value is a K value small enough to use approximation in a problem like this? Below 10^-3?

[Jayden Arevalo 2B]

Yes, usually to be safe, we say that K below 10^-4 we can approximate for the x, since it is very small compared to the initial concentrations.

[Tsii 2H]

Yes, in lecture I recall Professor Lavelle saying we can approximate when the number is less than 10^-3/10^-4 but I think it might be safer to just go by 10^-4.

[305589333]

I would stick to below 10^-4!

[JasmineReyes-2K]

Hi!,
I think generally we go based off if the value is less than 10^-4 !

[Julie Mai 1K]

Hello!

If the number is less than 10^-4, you can approximate.

[Madeline Ellmore 2C]

I believe he has said both 10^-3 and 10^-4, so to be safe, I would go with values below 10^-4. My TA said 10^-4 last discussion. Hope that helps!

[Ashley Johnson 2G]

Yes I clarified and it is 10^-4. Thank you everyone.

[Ainsley DeMuth 1H]

Initially we were told if is smaller than 1x10^-3, so to be safe, you can assume it is small enough if the value is smaller than 1x10^-4. However, if you make this assumption and want to check if it is valid using the percent ionization, I believe that is also possible since the percent ionization should be below 5% if the value truly was small enough.

[14b_student 2E]

If the number is less than 10^-4 you are allowed to approximate.

[505734174]

Usually, if K is less than 10^-4, it is safe to approximate but you should double check to make sure.

[Macy_Anderson_2F]

If the number is 10^-4 you are able to approximate and proceed with calculations. If you find that your percent ionization is greater than 5% using the approximated X value, then you need to redo the problem using the quadratic problem.

[tristenleem3B]

Generally if it is under 10^-4 then you should be good approximating it.

[Xzandalyn Kallstrom 2C]

In the lecture Dr. Lavelle said to only approximate numbers lower than x10^-4, any higher than that you should include them in the equation.

[Lauren Wasef 3C]

When the number is less than 10^-4 we can approximate!

[Evonne Chau 2I]

Hi Ashley,

During one of the lectures, Dr. Lavelle said we can approximate the value of X if it is smaller than 10^-3 or 10^-4. To be safe, I would go with 10^-4!

[Emily Quill 2H]

Anything below 10^-4 I think will be small enough.

[Jelix Tsan 2H]

To be safe, I would go with any K number that is less than 10^-4.

[Jaipal Virdi 2I]

Any k-value smaller than 10^-4 can be ignored when you approximate!

[ThomasL1L]

I believe anything under 10^4 should be okay.

[Mia Orr 3B]

When the k value is smaller than 10^-4, it should be okay to approximate.

[Jessica Li 1G]

Usually, when K is smaller than 10^-4 it is okay to approximate, or if x is less than 5% of the original concentration then approximation is valid.

[Kaitlyn_Urquilla_1I]

Hi! I believe that it if it smaller than 10^3/10^4 then we can approximate.

[Darren Apostol 2L]

That should be good.

[dahlia Faruque]

if the number is less than 10^-4 then you can approximate