5% Rule for Weak Acids

[Tasnia Haider 1E]

When we calculate the pH for weak acids, and the change in concentration is extremely small because kA is extremely small, do we automatically assume the change in concentration for the initial concentration is negligible like how we did the example in lecture? Do we always have to check using the 5% rule, making sure that the x value we calculate is lower than 5% of the original concentration?

[Chem_Mod]

Hi, Tasnia,
Yes, you can approximate change in the initial concentration to be negligible, if Ka is extremely small. However, you can use 5% rule if unsure whether the Ka is small enough. It will not hurt to check at the end whether your approximation is valid.

[Ramya Lakkaraju 1B]

How strict is the 5% rule? If the approximation is slightly more than 5% off, would we have to recalculate everything with more accuracy?