Which of the following will buffer near pH=9?

Question

a. 0.5M sodiumacetate/0.5M acetic acid
b. 0.5M ammonium chloride/0.5M ammonia
c. 0.5M sodium dihydrogen phosphate/0.5M hydrogen phosphate

i think a) 0.5M sodiumacetate/0.5M acetic acid will buffer near pH=9

Answer 1

No. Look at the Henderson-Hasselbalch equation.
pKa for acetic acid is about 4.7; it should be effective between 4.2-5.2 (actually +/- 1 but I moved it only 0.5 pH units).
pKa for NH3 is about 9.2 (8.7 to 9.7)
pK2 for H3PO4 is about 6.8 (6.3 to 7.3)