When I learned about pH approximations in school, I was told that each approximation fits to a certain acidity strength/pKa range. Those ranges are grouped in common categories (pKa borders are only approximately, as they change from book to book):
- very strong acids: $\mathrm{pK}_a < 0$
- strong acids: $0 \le \mathrm{pK}_a < 4$
- medium strong acids: $4 \le \mathrm{pK}_a < 8$
- weak acids: $8 \le \mathrm{pK}_a < 14$
- very weak acids: $\mathrm{pK}_a \ge 14$
From this point on, the arguments were made and approximations for certain strengths of acids were evaluated. But is it possible to choose the "right" approximation only based on acidity strength? Do those equations really only apply to it's categories?
This question originates in my wish to implement a program on my calculator that computes the pH for a given concentration and pKa value. I thought it might be wrong to simply ask for the pKa value and choose one of the approximations solely based on that.
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