Concentrations appearing in the Ka of buffer solutions

I've just finished studying buffer solutions and I think I understood more or less how they work, but there's something I can't make sense of:


In the $K\_a$ of equilibrium of a buffer solution made from Sodium acetate and acetic acid, we just have the concentrations of acid, $\ce{H+}$ and acetate ion. So, why isn't the concentration of $\ce{Na+}$ taken into consideration in the $K\_a$? Why just the others? 


Moreover as $[\ce{Na+}] = [\ce{CH3COO-}]$ (the acetate ion in solution is almost only the one dissociated from the salt) shouldn't in the Henderson-Hasselbach solution occur a $a^2$ factor? So in the numerator having $[\ce{CH3COO-}]^2$ and no just a single one?


Thank you




Because Na+ is a [spectator ion](https://en.wikipedia.org/wiki/Spectator_ion) in this situation. 


$$ \ce{Na+ + CH3COOH <=> Na+ + CH3OO- + H+}$$


In other words, Na+ is on both sides of the chemical equation, so [Na+] would appear in both the numerator and denominator of the equilibrium constant if Na+ is included. 


$[\ce{Na+}][\ce{CH3COO-}][\ce{H+}]/[\ce{Na+}][\ce{CH3COOH}] = [\ce{CH3COO-}][\ce{H+}]/[\ce{CH3COOH}]$


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Chemistry is based on the good-enough as opposed to theoretical perfection. Although $\ce{Na+}$ doesn't affect the pH directly, it does change the ionic strength of the solution. The pKa of acetic acid would be affected slightly by the ionic strength of the solution as a secondary correction. Textbook problems ignore such factors because it leads to calculation complications that require extra equations that in turn require iterative computer programs to solve. 


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