The base dissociation constant pKb is defined by which expression?

The base dissociation constant pKb is defined by which expression?

• A. log Kb
• B. -log Kb
• C. -log Ka
• D. 14 - pKb

Answer: (B)

Base strength for a base is described by its base dissociation constant, Kb. Expressing this with a negative logarithm gives pKb, defined as pKb = -log10(Kb). Using the negative log makes numbers easier to compare: a smaller pKb means a stronger base (larger Kb), while a larger pKb means a weaker base (smaller Kb). This scale also connects to pKa through pKa + pKb = pKw, which is about 14 at 25°C, so pKa = 14 − pKb. So the defining expression is the negative logarithm of Kb. The other options either omit the negative sign, refer to Ka (giving pKa, not pKb), or mix in a 14 − pKb relationship that actually corresponds to pKa, not the definition of pKb.

Base strength for a base is described by its base dissociation constant, Kb. Expressing this with a negative logarithm gives pKb, defined as pKb = -log10(Kb). Using the negative log makes numbers easier to compare: a smaller pKb means a stronger base (larger Kb), while a larger pKb means a weaker base (smaller Kb). This scale also connects to pKa through pKa + pKb = pKw, which is about 14 at 25°C, so pKa = 14 − pKb.

So the defining expression is the negative logarithm of Kb. The other options either omit the negative sign, refer to Ka (giving pKa, not pKb), or mix in a 14 − pKb relationship that actually corresponds to pKa, not the definition of pKb.