Buffer solutions help?

Hi I need help in buffer solutions. lets take this problem as an example: Calculate the PH of 0.200 M HC3H5O2 and 0.200 M NaC3H5O2. How do I know which one is the weak base or acid and which one is the conjugate base/acid? In the book it says its a weak acid with a conjugate base but I dont know how to get to that conclusion, please help. Thanks.