Buffer solution + equilibrium + Organic compound

[myvow]

For this question,why Ka =(ch3coo-)(h+)/(ch3cooh) instead of Ka =(ch3coo +x)(x)/(ch3cooh-x) ?

 

[epenguin]

This is your first post here, clearly two rather different homework problems, and to get help you will need to follow the guidelines https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/ regarding making an effort yourself first.

 

[Borek]

why Ka =(ch3coo-)(h+)/(ch3cooh) instead of Ka =(ch3coo +x)(x)/(ch3cooh-x)

First equation uses equilibrium concentrations, second uses initial concentrations modified by the extent of the dissociation, to calculate equilibrium concentrations. Do you understand what these means?

 

[epenguin]

For this question,why Ka =(ch3coo-)(h+)/(ch3cooh) instead of Ka =(ch3coo +x)(x)/(ch3cooh-x) ?

You have now modified your initial post. I do not understand what you have in mind by your x.
You might have been confused by the CH₃COONa. That is a conventional formula, it really just represents the composition of sodium acetate but in aqueous solution and for that matter in crystal sodium acetate is all Na⁺ and CH₃COO^-. When you add to this "HCl" - I.e. H⁺ + Cl^- - a fraction of the acetate ions CH₃COO^- become protonated to form CH₃COOH and then the calculations you are asked about involve that fraction, which actually determines the pH as you can see from the given formulae.