5% rule

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Nahelly Alfaro-2C
Posts: 59
Joined: Wed Nov 15, 2017 3:04 am

5% rule

Postby Nahelly Alfaro-2C » Sun Jan 20, 2019 10:10 pm

How does the 5% rule work? How do we know it should be used when calculating an equilibrium?

Lia Inadomi 1I
Posts: 62
Joined: Fri Sep 28, 2018 12:15 am

Re: 5% rule

Postby Lia Inadomi 1I » Sun Jan 20, 2019 10:19 pm

This is when approximating x while using the ICE table. For instance, if the equation becomes (x^2)/(.10-x)=K then you can approximate it to (x^2)/.10=K so you have to do easier calculations.

mahika_nayak_3L
Posts: 60
Joined: Fri Sep 28, 2018 12:26 am

Re: 5% rule

Postby mahika_nayak_3L » Sun Jan 20, 2019 10:25 pm

Additionally, if X is less than 5% of the initial concentration then the approximation made by eliminating X will be valid.

Griffin Carter 2I
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Joined: Wed Nov 14, 2018 12:23 am

Re: 5% rule

Postby Griffin Carter 2I » Sun Jan 20, 2019 10:25 pm

If the error of your approximation is less than 5% then using the approximation is fine. So you find your x value through the approximation method then divide by your initial amount of weak acid or base and multiply by 100. If the number calculated is greater than 5 then the quadratic formula should be used to solve for x. (x/[HA]) x 100 = some percent. Keep in mind this is the same formula for finding percent ionization of a weak acid or base.

Tony Ong 3K
Posts: 63
Joined: Fri Sep 28, 2018 12:23 am

Re: 5% rule

Postby Tony Ong 3K » Sun Jan 20, 2019 10:34 pm

Keep in mind that when approximating, Professor Lavelle said that it should be less than 10^-3 values. The 5% rule is simply a mechanism to check whether or not your approximations are valid.

George Ghaly 2L
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Been upvoted: 1 time

Re: 5% rule

Postby George Ghaly 2L » Mon Jan 21, 2019 3:00 pm

When dealing with weak acids or bases, the 5% rule can be applied to neglect the change of the reactants towards the products in order to make calculating the change significantly easier without use of the quadratic formula.

Kessandra Ng 1K
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Re: 5% rule

Postby Kessandra Ng 1K » Mon Jan 21, 2019 4:35 pm

When calculating the concentration of a reactant or product and given that Kc is less than 10^-3, you can disregard the x if it's something like "0.50-x" and approximate it to 0.50 only. When you get your final result, you then use the 5% rule to see if it is a valid approximation.

Rehan Chinoy 1K
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Joined: Fri Sep 28, 2018 12:25 am

Re: 5% rule

Postby Rehan Chinoy 1K » Mon Jan 21, 2019 5:03 pm

The 5% rule is useful because it allows us to simplify the equilibrium expression and avoid using the quadratic formula.

RachelCheung1A
Posts: 30
Joined: Fri Sep 28, 2018 12:19 am

Re: 5% rule

Postby RachelCheung1A » Mon Jan 21, 2019 5:07 pm

If the % pronotation is less than 5, then approximation (disregarding the initial change of x in the denominator) is valid.

Ashe Chen 2C
Posts: 31
Joined: Mon Jan 07, 2019 8:23 am

Re: 5% rule

Postby Ashe Chen 2C » Mon Jan 21, 2019 8:27 pm

If the percent protonation is <5%, then the approximation is valid and allows for finding x without using the quadratic formula

Kirsty Star 2H
Posts: 48
Joined: Fri Sep 28, 2018 12:24 am

Re: 5% rule

Postby Kirsty Star 2H » Tue Jan 22, 2019 10:55 am

If the K value is greater than 10^-3, that is when we cannot disregard x and have to solve using quadratic equation

Ricardo Martin 1J
Posts: 63
Joined: Fri Sep 28, 2018 12:15 am

Re: 5% rule

Postby Ricardo Martin 1J » Wed Jan 23, 2019 9:53 am

The 5% rule is just used to check if your approximation is valid. You can assume that you are allowed to approximate when the k value is <10^-3.


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