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In what cases can we assume that x makes a negligible difference when calculating change in concentration for a molecule in an equilibrium expression? When can we ignore it so that it makes the math of calculating change for other molecules in the reaction easier?
You can assume x makes a negligible difference when k<10^-3. Anything less than this indicates that the reaction strongly favors the reactants, so you can assume the change in concentration for the reactants is minuscule.
Going off that, is it safe to just never make that approximation? I understand there are problems where it'd be very difficult to solve without making the approximation to zero or using a graphing calculator but would we be deducted points for not utilizing this concept/trick?
Also, just as a way to check if it is reasonable that you ignored x: after you find the equilibrium concentrations of the reactants, find the percentage of ionization/protonization, which is the hydrogen ion concentration at equilibrium divided by the initial concentration of acid for finding the percentage of ionization. If this percentage is less than 5% then it is reasonable that you ignored x because this means that only a small amount of the acid gives of a proton and Kc is really small.
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