Question

1) Find the pH and fraction of dissociation (α) of the following:

a) 0.116 M solution of the weak acid HA with Ka = 1.16 ✕ 10−5. (Assume Kw = 1.01 ✕ 10−14. Enter unrounded values.)

b) 0.900 M solution of the weak base B with K_b = 9.00 ✕ 10⁻⁵. (Assume K_w = 1.01 ✕ 10⁻¹⁴. Enter unrounded values.)

Answer 1

a)

Ka = [H+][A-]/[HA]

[H+]= x= [A-]

[HA] = Mx = 0.116-x

1.16*10^-5 = (x*x)/(0.116-x)

x = 0.00154

since H+ = x = 0.00154

pH = -log(H+) = -log(0.00154 = 2.8124

Fraction dissociation = [H+] / M = 0.00154/0.116 = 0.0132758

b)

similar

Kb = [HB+][OH-]/[B]

[HB+]= x= [OH-]

[B] = M - x = 0.9-x

9*10^-5 = (x*x)/(0.9-x)

x = 0.008955

since Oh-= x = 0.008955

pOH = -log(OH-) = -log(0.008955) = = 2.047

pH = 14-pHO =! 4-2.047 = 11.953

Fraction dissociation = [OH-] / M = 0.008955/0.9= 0.00995