ap chem acid and base

Question

Using data from this table of dissociation constants for acids, calculate [OH- ] and pH for each of the following solutions. (Kw = 1.0 10-14.)

 

A mixture that is 0.050 M in CH3COONa and 0.044 M in (CH3COO)2Ba

 

Find:

[OH- ] 
pH

Answer 1

Both sodium acetate and barium acetate are strong electrolytes, therefore [CH₃COO^-1] = 0.050 +(0.044x2) = 0.138 M (assuming no change in volume).

acetate ion is the conjugate base of a weak acid, acetic acid, Ka = 1.8x10^-5, therefore, reacts with water as a weak base (Kb = Kw/Ka = 5.5₆x10^-10

CH₃COO^1-_(aq) + H₂O_(l) ⇔ CH₃COOH_(aq) + OH^1-_(aq)

setting up the ICE table:

IC 0.138M, 0, 0

CC. -x, +x, +x

EC. (0.138-x), x, x

5.56x10^-10 = x²/ (0.138 - x)

assuming x « 0.138

x = [OH^1-] = 8.75x10^-6( water contribution to OH^1- contraction is negligible, and the assumption x« 0.138 is valid)

pOH = 5.06 ,therefore, pH = 8.94