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You can assume x is small when the equilibrium constant (K) is less than 10^-3 as a general rule of thumb. But to test it, after you calculate the equilibrium concentration, you can do the equilibrium concentration of the conjugate base divided by the initial concentration of the weak acid times 100 (percent dissociation/protonation) and if it is less than 5%, then your assumption is valid.
When the K value is less than 10-3, it is generally safe to estimate without the x. However, make sure to only exclude variables that are being added or subtracted. To make sure estimation was appropriate, make sure the percent dissociation is under 5%.
205192823 wrote:When solving for concentrations with the ice table, in what situation can you assume x is too small and therefore can be excluded from the equation? EX: 0.2-x, in this situation would I just disregard x and just keep 0.2?
When K is less than 10^-3, then you can disregard the change in concentration of the reactants (usually -x), as such a little number of moles of reactant were converted to product.
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