Answer:
pKb = 10.96
Explanation:
Tartaric acid is a dyprotic acid. It reacts to water like this:
H₂Tart  +  H₂O  ⇄  H₃O⁺   +  HTart⁻         Ka1
HTart⁻  +  H₂O  ⇄  H₃O⁺   +  Tart⁻²           Ka2
When we anaylse the base, we have
Tart⁻²   +  H₂O  ⇄  OH⁻  +  HTart⁻       Kb1
HTart⁻  +  H₂O  ⇄  OH⁻  +    H₂Tart          Kb2
Remember that Ka1 . Kb2  = Kw, plus pKa1 + pKb2 = 14
Kb2 = Kw / Ka1  →   1×10⁻¹⁴ / 9.20×10⁻⁴  = 1.08×10⁻¹¹
so pKb = - log Kb2   → - log 1.08×10⁻¹¹ = 10.96