Taking x away in a Ka calculation

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Dina Geotas 4A
Posts: 61
Joined: Fri Sep 28, 2018 12:29 am

Taking x away in a Ka calculation

Postby Dina Geotas 4A » Wed Jan 16, 2019 2:30 pm

Why are we able to disregard the -x when calculating the Ka from an ICE chart?

Kristen Kim 2K
Posts: 70
Joined: Fri Sep 28, 2018 12:16 am

Re: Taking x away in a Ka calculation

Postby Kristen Kim 2K » Wed Jan 16, 2019 3:02 pm

This only applies if the change in composition (x) is less than 5% of the initial value. Usually if Ka is less than 10^-3, I would disregard the x.

JiangJC Dis2K
Posts: 72
Joined: Fri Oct 12, 2018 12:16 am

Re: Taking x away in a Ka calculation

Postby JiangJC Dis2K » Wed Jan 16, 2019 3:57 pm

If you are calculating ka from given concentrations, so the ka is unknown, you employ the 5% rule. You would first calculate the percent ionization by dividing the equilibrium concentration by the initial concentration of one of the products times 100 in order to determine the change in x. If this answer is less than 5%, then the approximation is ok.

daniella_knight1I
Posts: 57
Joined: Fri Sep 28, 2018 12:18 am

Re: Taking x away in a Ka calculation

Postby daniella_knight1I » Wed Jan 16, 2019 4:47 pm

If the x in Ka is so small it's insignificant, (less than 5%) then you can disregard it.

Miya Lopez 1I
Posts: 62
Joined: Tue Nov 14, 2017 3:00 am
Been upvoted: 1 time

Re: Taking x away in a Ka calculation

Postby Miya Lopez 1I » Wed Jan 16, 2019 4:51 pm

In the example Dr. Lavelle did in class today with kA to find the pH, can someone help explain to me how he was able to approximate it to ?

Emily Ng_4C
Posts: 65
Joined: Fri Sep 28, 2018 12:17 am

Re: Taking x away in a Ka calculation

Postby Emily Ng_4C » Wed Jan 16, 2019 5:12 pm

Sometimes, the x is so small that it doesn't have an effect on the overall concentration. For instance, let's say the initial concentration is 1 x 10^-1 and the K value is extremely small, we would know that the numerator/reactants would also have to be extremely small so that the ratio will equal the K value. Thus, the x will not have an effect on the initial concentration (comparatively).


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