Why Should We Care?
Quantitatively expressing the strengths of acids and bases in Ka, pKa, Kb and pKb values provides chemists with a quick means of evaluating the strength of an acid or base. This information provides a simple indication of behavior and properties of the acid or base. These indicators of strength also alert chemists to the potential dangers of using an acid or base in the laboratory, helping chemists identify what precautions may be required.
Worked Example
Weak acids in the ocean are decreasing ocean pH and resulting in harmful consequences for the health of marine ecosystems. The strengths of weak acids are expressed both as Ka and pKa. Test your understanding of the connection between Ka and pKa by answering the following questions:
The Ka for aquated benzoic acid, C6H5COOH(aq), is 6.3×10-5 at 25°C. Calculate pKa.
Use the following equation:
Aquated chloroacetic acid, ClCH2COOH(aq), has a pKa of 2.87. Is aquated chloroacetic acid a stronger or weaker acid than aquated benzoic acid?
The pKa of aquated benzoic acid is 4.20 while the pKa of aquated chloroacetic acid is 2.87. Because a smaller pKa indicates a greater extent of reaction with water, aquated chloroacetic acid is the stronger acid.
Your Turn
The strength of weak acids can be expressed as Ka or pKa values. The pKa of epinephrine hydrochlorine is 9.53. What is the Ka of this acid?
The pKa of aquated benzoic acid is 4.20 while the pKa of aquated chloroacetic acid is 2.87. Because a smaller pKa indicates a greater extent of reaction with water, aquated chloroacetic acid is the stronger acid.
