The pH of a buffer, calculated by the method given in introductory texts, is so dramatically inaccurate that it is dangerous. If the calculation is to be taught, then it must use either conditional constants or (for more advanced students) activity coefficients. It would be better not to teach the calculation at all, but to concentrate on the qualitative description of buffers, buffering, and buffer capacity.
Stephen J. Hawkes (Sat,) studied this question.