Using the reaction HB H+ + B- and the Ka for that expression, calculate the Ka given the initial concentration of weak acid to be 0.150 M and the pH of the solution to be 2.79.

Question

Using the reaction HB<=> H+ + B- and the Ka for that expression, calculate the Ka given the initial concentration of weak acid to be 0.150 M and the pH of the solution to be 2.79.

DrBob222 · Answer

pH = -log)(H^+) = 2.79
(H^+) = 1.5E-3 but you need to do that more accurately. My number is an estimate.
............HB ==> H^+ + B^-
initial....0.150...0......0
change...-1.5E-3..1.5E-3..1.5E-3
equil..0.148.....1.5E-3..1.5E-3

Ka = (H^+)(B^-)/(HB)
Substitute and solve for Ka.