9. Consider a solution of formic acid (HCHO2) and sodium formate (NaCHO2), at pH=3.70. After the addition of 0.015 moles of HNO3, the pH decreases by 0.12. What are the initial molarities of HCHO2 and CHO2-, if the Ka of HCHO2 is 1.8x10-4?

Equilibrium system:

HCHO2 (aq) H+ (aq) + CHO2- (aq)

Calling the concentration of HCHO2 x, and the concentration of CHO2- y, the correct Henderson-Hasselbach equation which describes the buffer before the addition of HNO3 is:

a) 3.70 = 3.58 - log {y/x}
b) 3.58 = 3.74 + log {y/x}
c) 3.70 = 3.74 + log {x/y}
d) 3.70 = 3.74 - log {x/y}