5% approximation for weak acids/bases

[gwynlu1L]

hello,
I know today in lecture Lavelle talked about how in weak acid/base equilibrium questions we ca make the approximation that x is not significant enough so we can simplify the quadratic and make solving for x easier, as long as x is less than 5% of the initial. can we assume this pretty much for all weak acid/base equilibrium questions, and if not is there a way to check this approximation is valid before going through the calculations instead of checking the percent ionization after using the approximation?

[Tarika Gujral 1K]

The 5% approximation can be used for values of K that are very large or very small.

If K is large — P+x = P
If K is small — R-x = R

But the verification must be computed after calculating equilibrium values.

[Samantha Ito 2E]

Lavelle said that if the K value is less than 10^-3, then typically we can use the approximation.