Pivot Point Chemistry Practice Test

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What is pKa and how does it relate to acid strength?

pKa is the same as Ka; higher pKa means stronger acid

pKa is the negative log of Ka; lower pKa means a stronger acid.

The key idea is how pKa relates to how easily an acid donates a proton. Ka measures the extent of dissociation; a larger Ka means the acid donates protons more, which is a stronger acid. pKa is defined as the negative base-10 logarithm of Ka, pKa = -log10(Ka). Because of this logarithmic relationship, a bigger Ka (stronger acid) gives a smaller pKa. Therefore, a lower pKa indicates a stronger acid.

That’s why the statement describing pKa as the negative log of Ka and linking a lower pKa to greater acidity is the best fit. The other ideas aren’t correct because pKa is tied to acid strength (not base strength), it’s not simply equal to Ka, and it does relate to how strong an acid is.

pKa measures base strength

pKa has no relation to acid strength