Moderators: Chem_Mod, Chem_Admin

Selena Yu 1H
Posts: 108
Joined: Fri Aug 09, 2019 12:16 am
Been upvoted: 1 time


Postby Selena Yu 1H » Thu Jan 16, 2020 12:37 am

Why is it that if X is less than 5% of initial then approximation is okay? Like why is the cut off 5%?

Ronak Naik
Posts: 103
Joined: Sat Jul 20, 2019 12:15 am

Re: Approximation

Postby Ronak Naik » Thu Jan 16, 2020 12:41 am

I think there is no official reason why it is 5%; however, the approximation check is used to make sure that X is such a small number that it allows to consider it insignificant when add or subtracting to another number.

Sanjana K - 2F
Posts: 102
Joined: Sat Sep 07, 2019 12:17 am
Been upvoted: 1 time

Re: Approximation

Postby Sanjana K - 2F » Thu Jan 16, 2020 9:21 am

In class, he also said that if your Ka is less than 10^(-3), then you can generally approximate by ignoring the "-x" term but you might want to justify it on the test by explaining that since x is less than 5% of the initial concentration, it's considered a negligible difference so it can be "ignored."

Posts: 36
Joined: Tue Feb 26, 2019 12:15 am

Re: Approximation

Postby Alondra » Sun Jan 19, 2020 5:57 pm

I believe this has to do with the value being so small that it barely even causes a minimal change on a bigger initial concentration.

J Medina 2I
Posts: 102
Joined: Wed Sep 25, 2019 12:17 am

Re: Approximation

Postby J Medina 2I » Sun Jan 19, 2020 6:43 pm

Does it have to do with using significant figures? Like when we round our answer to a significant figure, the difference in our answer that a -X would have provided does not even make a difference in our final rounded answer?

Posts: 104
Joined: Sat Aug 17, 2019 12:17 am

Re: Approximation

Postby TimVintsDis4L » Sun Jan 19, 2020 6:50 pm

I doubt there's any science behind it being 5%. Scientists are lazy so they just choose a good number to go by and stick with that. The 5% is just something for us to follow as to whether or not to use the quadratic formula.

Charysa Santos 4G
Posts: 107
Joined: Wed Sep 18, 2019 12:21 am

Re: Approximation

Postby Charysa Santos 4G » Sun Jan 19, 2020 7:16 pm

It's just a general rule to check whether or not your approximation is valid because the amount (X) should be small enough that it wouldn't profoundly impact your equilibrium expression.

Return to “Calculating pH or pOH for Strong & Weak Acids & Bases”

Who is online

Users browsing this forum: No registered users and 1 guest