When to use the approximation  [ENDORSED]

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Christian_Makar_2D
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Joined: Wed Sep 21, 2016 2:57 pm

When to use the approximation

Postby Christian_Makar_2D » Mon Nov 21, 2016 2:44 pm

When using the ICE box to calculate the concentrations of a weak acid or base, I know that we can use an approximation in the calculation of the acidity or basicity constant if the constant is small. But what is the exact of when to to use this approximation?

Palmquist_Sierra_2N
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Re: When to use the approximation

Postby Palmquist_Sierra_2N » Mon Nov 21, 2016 2:49 pm

Whenever x is smaller than 10^-3.
You also know the approximation is okay if x is less than 5% of the initial and you find this by using the formula for % ionization of the acid.

Chem_Mod
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Re: When to use the approximation  [ENDORSED]

Postby Chem_Mod » Mon Nov 21, 2016 2:53 pm

Okay if K is 10-5 or smaller.

If unsure, K ~ 10-4, then calculate % ionization (or % protonation, or % product yield). If less than 5% then approximation okay.

I discussed all of this in class today.


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