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You only ignore X when it is being added or subtracted from a number, because X is so small that it barely makes any difference to the resulting value. This occurs when K is less than 10^-3. If X is by itself or being squared on the numerator of a fraction, don't assume X is 0 and instead continue solving for that X value.
We can only ignore x in an ice table when the equilibrium constant, K, is less than 10^-4. Also, we are not necessarily saying x is 0. We can neglect the x because the value of x is so small that it makes very minimal difference when subtracting from the initial concentration. For example, if the initial concentration is 0.5 and x is 1x10^-7, subtracting 0.5 is essentially just 0.5 since the x is very small.
You can also check for this by finding the percent protonization by taking the concentration you get and dividing it bby the original concentration of your acid/base. If it's less than 5% then treating x as being so small that it is insignificant is valid.
everything above^^^ and you can check the 5% rule by taking the value you plug into the -log for pH and dividing it by the original starting concentration of whatever weak acid was used in the problem (usually a given) then you multiply it by 100%
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