What is the pH of a solution that results from diluting 0.30 mol acetic acid (CH3CO2H) and 0.20 mol sodium acetate (NaCH3CO2) with water to a volume of 1.0 L? (Ka of CH3CO2H = 1.8 x 10^-5? I know to find the molarity of each, considering the 1 L volume. I know to use the Henderson-Hasselbalch equation. My question is, how do I know which compound to put as the numerator in the equation: pH = pKa + log [NaCH3CO2]/[CH3CO2H]...I know this is the correct way to do it, but I need to know WHY sodium acetate is the numerator and not acetic acid