All possible reactions between planned reagents that could lead to buffer solution by fractional neutralization. Look up the suitable equilibrium constants and find out the central pH of each possible buffer.
0.2 M sodium formate [Na(CHO2)]
0.2 M potassium hydrogen phosphate [ K2HPO4]
0.2 M ammonium chloride [ NH4Cl]
0.2 M hydrochloric acid
0.2 M sodium hydroxide