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Acids, Bases, and Buffers

Manual: General · Subject: Chemistry

Analyze acid-base strength, $pH$, buffer action, and titration logic.

Proton chemistry

Acid-base definitions

In the Brønsted-Lowry model, acids donate protons and bases accept protons. Strong acids and bases dissociate nearly completely in water, while weak acids and bases establish equilibria.

pH and pOH

The acidity of a solution is measured by pH=−log⁡[H+]\mathrm{pH} = -\log[H^+] and pOH=−log⁡[OH−].\mathrm{pOH} = -\log[OH^-]. At 25∘C25^\circ\mathrm{C}, pH+pOH=14.\mathrm{pH} + \mathrm{pOH} = 14.

ℹ️

Strong vs weak

Strength is about the extent of dissociation, not concentration. A dilute strong acid is still a strong acid.

Buffer action

  1. 1

    Step 1: Identify the weak acid/base pair and its conjugate partner.

  2. 2

    Step 2: Add a small amount of acid or base.

  3. 3

    Step 3: The conjugate species consumes the added acid/base.

  4. 4

    Step 4: The pH changes only slightly because the ratio of pair components changes modestly.

What is the pH of a solution with [H+]=1.0×10−3 M[H^+] = 1.0\times10^{-3}\,\mathrm{M}?

What makes a buffer solution effective?

Acid-base pairs

Conjugate acid-base pair

  • Differ by one proton
  • Related by proton transfer

Buffer pair

  • Weak acid/base with conjugate partner
  • Resists pH change

A titration curve for a weak acid with a strong base typically has