5% rule

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Junwei Sun 4I
Posts: 125
Joined: Wed Oct 02, 2019 12:16 am

5% rule

Postby Junwei Sun 4I » Thu Jan 23, 2020 12:06 am

When do we use the 5% rule? Do we only use it in acid and base equilibrium questions or any equilibrium questions?

Hope Hyland 2D
Posts: 50
Joined: Wed Feb 20, 2019 12:16 am

Re: 5% rule

Postby Hope Hyland 2D » Thu Jan 23, 2020 12:08 am

I think you use it anytime you make an approximation, or assume that x is so small that it's essentially equal to 0.

Jordan Young 2J
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Joined: Thu Jul 25, 2019 12:17 am
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Re: 5% rule

Postby Jordan Young 2J » Thu Jan 23, 2020 12:10 am

You use it to check if you can appropriately approximate x to be a small enough that it would effect the initial concentration that much when using the calculation involving K. You will most likely use this when solving equilibrium equations.

Matthew Tsai 2H
Posts: 101
Joined: Wed Sep 18, 2019 12:20 am

Re: 5% rule

Postby Matthew Tsai 2H » Thu Jan 23, 2020 12:13 am

Also keep in mind x cannot be approximated to zero if it is by itself. So values like x, x^2, etc. would be kept but values like 0.1-x, 0.1+x, etc. can be approximated to 0.1

Alexandra Bartolome 2H
Posts: 102
Joined: Sat Jul 20, 2019 12:17 am

Re: 5% rule

Postby Alexandra Bartolome 2H » Thu Jan 23, 2020 3:17 am

I believe when K is less than 10^-3, you can approximate with the 5% rule.

JohannaPerezH2F
Posts: 94
Joined: Wed Sep 18, 2019 12:18 am

Re: 5% rule

Postby JohannaPerezH2F » Thu Jan 23, 2020 9:20 am

this rule is used to calculate how accurate your approximation for x is. if it is less than 5% you're good

LNgo 1G
Posts: 100
Joined: Sat Aug 24, 2019 12:16 am

Re: 5% rule

Postby LNgo 1G » Thu Jan 23, 2020 9:22 am

I believe you can use it whenever you are calculating K using ICE table just as long as K<10^-3

kristi le 2F
Posts: 102
Joined: Thu Jul 11, 2019 12:15 am

Re: 5% rule

Postby kristi le 2F » Thu Jan 23, 2020 11:42 am

The % ionization is the concentration of the molecule in the ionized state divided by the concentration of the molecule in the unionized state. If this is less than 5%, not a significant amount of the molecule ionized compared to the initial concentration.

Abby Soriano 1J
Posts: 103
Joined: Sat Aug 24, 2019 12:16 am

Re: 5% rule

Postby Abby Soriano 1J » Thu Jan 23, 2020 11:55 am

Whenever you do any calculations involving ICE tables, you can use the 5% rule to ensure that your approximation that x is small is valid.

Mariana Fuentes 1L
Posts: 43
Joined: Wed Nov 15, 2017 3:00 am

Re: 5% rule

Postby Mariana Fuentes 1L » Thu Jan 23, 2020 12:33 pm

I believe you use it when the number is very close to zero that it doesn’t make a difference to the overall answer

Shivam Rana 1D
Posts: 106
Joined: Fri Aug 09, 2019 12:16 am

Re: 5% rule

Postby Shivam Rana 1D » Thu Jan 23, 2020 12:35 pm

When you cancel out the x and solve. You see if the x value is less than 5% afterwards.

Simon Dionson 4I
Posts: 107
Joined: Sat Sep 14, 2019 12:17 am

Re: 5% rule

Postby Simon Dionson 4I » Thu Jan 23, 2020 12:59 pm

In my experience, I usually use this rule in weak acid/base equilibria.

Dina 2k
Posts: 57
Joined: Wed Sep 18, 2019 12:17 am

Re: 5% rule

Postby Dina 2k » Thu Jan 23, 2020 1:29 pm

is the formula to check to see if the answer is in the 5% range x/[HA]x100% (given its an acid). and isn't this the same formula to find percent deprotonated?

Pablo 1K
Posts: 118
Joined: Sat Feb 02, 2019 12:15 am

Re: 5% rule

Postby Pablo 1K » Thu Jan 23, 2020 1:34 pm

You use that rule when x is less than 10^-3, its so small it wont change the concentration of a molecule.


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