a solution that is 0.51 M CH3COOH and 0.28 M CH3COONa
is that a buffer?
when can u tell when to us the henderson hasslebach
Yes, its a buffered solution. Use the H-H equation when you have a buffer. That's always a weak acid and its salt (conjugate base) or a weak base and its salt (conjugate acid).
In this case you have a weak acid and sodium acetate which is the salt. NH3 and NH4Cl would be an example of a weak base and its salt.