Calculate the pH of the buffer that results from mixing 57.9 mL of a 0.278 M solution of HCHO2 and 17.1 mL of a 0.535 M solution of NaCHO2. The Ka value for HCHO2 is 1.8×10−4.

Since there is the same amount of agn03 in both silver chloride and silver nitrate they produce the same amount of moles. therefore silver chloride produces the same amount of silver nitrate, so your answer is 15.0 moles of silver nitrate. i hope this !