5% rule

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5% rule

Postby Shanzey » Sun Jan 19, 2020 8:48 pm

When approximating, how do we test to make sure that our approximation violates the 5% rule? Do we have to do (equilibrium concentration)/(initial concentration)?

Ariana Iranmahboub1G
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Re: 5% rule

Postby Ariana Iranmahboub1G » Sun Jan 19, 2020 8:58 pm

Yes, you calculate (equilibrium concentration)/(initial concentration) times 100 and if it is less than 5%, the approximation is valid.

Bryce Ramirez 1J
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Re: 5% rule

Postby Bryce Ramirez 1J » Sun Jan 19, 2020 9:08 pm

If the value you get is more than 5%, you would know because the value should be significantly higher so that you are sure that it's not a valid approximation. If it's close around the 5% range, then you should redo the problem and make sure not to round any of the numbers until the final answer.

Brian J Cheng 1I
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Re: 5% rule

Postby Brian J Cheng 1I » Sun Jan 19, 2020 9:10 pm

If the equilibrium concentration divided by the initial reactant concentration is less than 0.05, then the equilibrium constant is valid. Basically this means: the change in concentration of product can be ignored while solving the equilibrium constant equation as long as it is less than 5% of the initial starting concentration.

Long Luong 2H
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Re: 5% rule

Postby Long Luong 2H » Sun Jan 19, 2020 10:22 pm

Do you only check with the 5% rule if you make the assumption about ignoring the change in concentration of product? In other words, if you solve chemical equilibrium problems with the quadratic formula, do you check with the 5% rule?

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Re: 5% rule

Postby Owen-Koetters-4I » Sun Jan 19, 2020 10:29 pm

You calculate initial concentration/equilibrium concentration by plugging in your approximated x-value. If this value is greater than 0.05 you must solve the cubic equation.

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Re: 5% rule

Postby KDang_1D » Sun Jan 19, 2020 11:11 pm

Yes, that expression refers to "percent protonation." Take note that these are the concentrations of the "primary" compounds involved in the reaction. For example, regarding NH3 + H2O --> NH4+ + OH-, you would only use [NH4+] formed /[NH3] initial, not including [H2O] and [OH-]

Also note that if the reaction equation has stoichiometric coefficients, the concentration of the product will be the x-value (from an ICE table) multiplied by that number. For example: HA + H2O --> 2A- + H3O; [A-] = 2x

Jacob Villar 2C
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Re: 5% rule

Postby Jacob Villar 2C » Sun Jan 19, 2020 11:19 pm

It’s equilibrium concentration divided by initial concentration.

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Re: 5% rule

Postby 205154661_Dis2J » Sun Jan 19, 2020 11:22 pm

We test it out by dividing equilibrium concentration/initial concentration x 100. if the value is less than 5%, the approximation is valid.

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