Determining pH

[degenha7]

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When determining pH of weak acid/base do I always use the formula K=[pdt]/[rct]?

When do I use the Henderson-Hasselbach equation? I know it is for buffers, but I keep confusing the two...

 

[babowc]

I use [H+] = sqrt ([Ka][HA]), derived from your equation.
Then pH = -log[H+]

Henderson-Hasselbach should be used when you've a weak acid+conjugate base or weak base+conjugate acid.