Which combination of the following substances would make

Question

Consider the following acids and bases:

HCO₂H, K_a=1.8x10^-4

HONH_2, K_b=1.1x10^-8

HOBr, K_a=2.0x10^-9

(C₂H₅)₂NH, K_b=1.3x10^-3

Which combination of the following substances would make the best choice to make a pH=6.0 buffer solution?

HOBr

KCO₂H 

NaOBr

HONH₂

(C₂H₅)₂NH

HONH₃NO₃

(C₂H₅)₂NH₂Cl

HCO₂H

A best buffer has approximately equal concentrations of weak acid and conjugate base (or weak base and conjugate acid). When [weak acid] = [conjugate base], then pH = pK_a of the acid component of the buffer. Therefore, look for a weak acid/conjugate base pair or weak base/conjugate acid pair whose pK_a value for the acid component in the buffer is close to the pH you want to make the buffer at.