pKa is defined as which logarithmic expression?

pKa is defined as which logarithmic expression?

• A. -log Ka
• B. log Ka
• C. pKa = 14 - pKb
• D. pKb

Answer: (A)

The concept being tested is how pX scales are defined as negative base-10 logarithms of equilibrium constants. For acids, the pKa is defined as the negative logarithm of the acid dissociation constant: pKa = -log10(Ka). This form makes pKa a convenient, positive number for typical weak acids (since Ka is less than 1, the negative log gives a value above 0). It also links acidity to strength: smaller pKa means stronger acid because a larger Ka yields a smaller pKa, and a larger pKa means weaker acid. The other options don’t fit the direct definition. Using log Ka would omit the minus sign, which is essential to get the conventional p-scale values. pKb is a different quantity (the base dissociation constant on a negative log scale), not a direct expression for pKa. The relation pKa = 14 − pKb arises from the identity pKw = 14 at 25°C, relating conjugate acid–base pairs, but it is a derived connection, not the definition of pKa itself.

The concept being tested is how pX scales are defined as negative base-10 logarithms of equilibrium constants. For acids, the pKa is defined as the negative logarithm of the acid dissociation constant: pKa = -log10(Ka). This form makes pKa a convenient, positive number for typical weak acids (since Ka is less than 1, the negative log gives a value above 0). It also links acidity to strength: smaller pKa means stronger acid because a larger Ka yields a smaller pKa, and a larger pKa means weaker acid.

The other options don’t fit the direct definition. Using log Ka would omit the minus sign, which is essential to get the conventional p-scale values. pKb is a different quantity (the base dissociation constant on a negative log scale), not a direct expression for pKa. The relation pKa = 14 − pKb arises from the identity pKw = 14 at 25°C, relating conjugate acid–base pairs, but it is a derived connection, not the definition of pKa itself.