[Alexander]

As this famous equation states:

pH=pK+lg([base]/[acid])

Now let us take sample. Phosphate buffer with pH=7. You can find anywhere that for this case you need the ratio of K₂HPO₄/KH2PO4=61.5/38.5. and this values are correct. I have done myself. (base => K₂HPO₄)

pK in this case, as you can also find almost everywhere is 7.21

therefore:
lg([base]/[acid])=-0.21
and
[base]/[acid]=10^-0.21=0.616595...

Please note that this value is reverse of above mentioned ratio. The proper (above given) ratio you can get if pK=6.79.
well, at some places I also found that pK is 6.86 but those sources are less reliable and anyway, this value does not lead to proper solution.

Any Ideas?

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients

[RandoFlyer]

Isn't the formula pH=pK +log ([conj/ base]/[acid])  ?