sodium propanoate

[Guest kaytiero]

I'm doing questions on acids and bases, and I need to work out the pH of a mixture of sodium hydroxide and propanoic acid. I know the concentrations of each, and I know the number of moles of unreacted propanoic acid and its concentration.

The question tells me to use the equation

pH=pKa - log [acid]/[base]

I'm ok til I get to the pKa bit, do I use the pKa of propanoic acid which I know to be 1.3x10-5, or do I use one for sodium propanoate? My chem teacher said to look up the appropriate pKa value but as I don't know which to use I'm a bit stuck, plus I can't find a pKa value for sodium propanoate anywhere.

Someone help

[chemistry]

The a in the subscript of Ka indicates acid.