posted by Reven .
Determine the pH (to two decimal places) of a solution prepared by adding 15.9 mL of 0.0551 M sodium hydrogen oxalate (NaHOOCCOO) to 0.488 g of sodium oxalate (Na2OOCCOO).
Hydrogen Oxalate pKb of 12.77
Oxalate pKb of 9.81
mmoles acid = 15.9 x 0.0551 = ?
mmoles base = 0.488/molar mass Na2C2O4)
pH = pKa2 + log(base)/(acid)
You must convert pKb above (you want the HC2O4^- to pKa by pKa + pKb = pKw = 14.
Also, I have not taken the volume into account and used ONLY millimoles. Since that is a ratio, mmoles/mmoles cancels out the units AND since concn is mmoles/mL and the mL is the same for acid and base, the volume (whatever it may be) always cancels, too.