A "rule of thumb" that is useful in determining whether x is less than 5% of the initial concentration of the acid is to determine whether one thousand times the $K$_a is less than the initial concentration of the acid. If that is true, then x will be less than 5% of the initial molarity of acid and simplification is acceptable.

If $K$a x 1000 < initial molarity, simplify! If $K$a x 1000 > initial molarity, DO NOT SIMPLIFY!!

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