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5% rule

Posted: Mon Dec 04, 2017 6:29 pm
by Peter Dis1G
What is specific about 5% rule in calculating chemical equilibrium of Kw?

Re: 5% rule

Posted: Mon Dec 04, 2017 6:38 pm
by Curtis Tam 1J
Basically it's used to determine whether or not you can use approximation when solving for x in an ice table. For example, if the percent deprotonation is less than 5, then it is okay to write for example in the denominator that 0.1-x is essentially 0.1.

Re: 5% rule

Posted: Tue Dec 05, 2017 12:26 pm
by Sheel Shah 1H
Exactly what the above response said; however, another useful tool you could use is if the Ka/Kb value is less than 10x10^-3, it is generally safe to assume the change in x as negligible (for the initial reactant). This is NOT to say that x=0, it still changes by some amount, just an incredibly small amount. You would still calculate x for the products side, just that in the reactants the difference in the original value and new value are typically the same when x is incredibly small.