Unit 8 · Topic 8.8 Beta

Properties of Buffers

A buffer resists pH change because its two components react with whatever is added: the conjugate base takes up added acid and the weak acid takes up added base.

Practice 1: Models and RepresentationsPractice 4: Model AnalysisPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Pour 1 mL of strong acid into a glass of water and its pH drops by five units. Pour the same acid into a glass of acetate buffer and the pH barely moves, by less than a tenth. Nothing magic happens: the acid is still added, but particles in the buffer react with it before it can pile up.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which pair makes a buffer?

  1. HNO₂ and NaNO₂
  2. HCl and NaCl
  3. NaOH and NaCl
  4. HNO₃ and NaNO₃
Show the answer

A weak acid and its conjugate base; HNO₂ is weak.

  • Correct: HNO₂ and NaNO₂:
  • HCl and NaCl:
  • NaOH and NaCl:
  • HNO₃ and NaNO₃:

2. For an acid with pKa 4.7, which form predominates at pH 5.7?

  1. A⁻, about 10 to 1
  2. HA, about 10 to 1
  3. Equal amounts
  4. A⁻, about 1.2 to 1
Show the answer

pH is 1 unit above pKa, so [A⁻]/[HA] = 10.

  • Correct: A⁻, about 10 to 1:
  • HA, about 10 to 1:
  • Equal amounts:
  • A⁻, about 1.2 to 1:

Part 4 · See it

See it first

Bars for a 1.00 L buffer: 0.100 mol HA and 0.100 mol A⁻ before (pH 4.74); after adding 0.010 mol OH⁻, 0.090 mol HA and 0.110 mol A⁻ (pH 4.83). The same hydroxide in 1.00 L of pure water raises the pH from 7.00 to 12.00.
The same 0.010 mol of hydroxide changes the buffer's pH by 0.09 units and pure water's by 5.00. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Strong acid is added to a bufferH₃O⁺ + A⁻ → HA + H₂O runs to completion, so the added H₃O⁺ does not stay free
  2. Strong base is added to a bufferOH⁻ + HA → A⁻ + H₂O runs to completion, so the added OH⁻ does not stay free
  3. Each addition only shifts a little HA into A⁻ or backthe ratio [A⁻]/[HA] changes slightly, so [H₃O⁺] = Ka × [HA]/[A⁻] changes slightly
  4. Diluting a buffer lowers [HA] and [A⁻] by the same factorthe ratio, and so the pH, stays almost the same

Part 6 · Key ideas

Key ideas

  • Buffer action: added acid is consumed by A⁻; added base is consumed by HA. The added ion is turned into a member of the pair.
  • Buffer pH depends on the ratio [A⁻]/[HA], not on the amounts. Same ratio, same pH.
  • Diluting a buffer barely changes its pH.
  • A buffer limits pH change; it does not stop it. Each addition shifts the ratio a little.

Part 7 · Misconception

A common mistake

The wrong idea: A more concentrated buffer has a lower pH because it contains more acid.

What actually happens: It also contains more conjugate base. The pH depends on the ratio [A⁻]/[HA], so two buffers with the same ratio have the same pH; the concentrated one can simply absorb more added acid or base.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Data table

Adding acid to three solutions

A student adds 1.0 mL of 1.0 M HCl (1.0 × 10⁻³ mol) to 100.0 mL of each solution below and records the pH before and after. The buffer uses acetic acid (pKa = 4.74).

pH before and after adding 1.0 × 10⁻³ mol HCl
Solution (100.0 mL)pH beforepH after
Pure water7.002.00
0.10 M CH₃COOH + 0.10 M CH₃COONa4.744.65
0.10 M NaCl7.002.00

1. Which claim do the data support?

  1. The acetic acid / acetate mixture resists a pH change
  2. The three solutions resist a change in pH to the same extent
  3. NaCl solution is a buffer because it contains ions
  4. Water resists a change in pH better than the mixture
Show the answer

The mixture's pH moved by 0.09 units; water and NaCl solution each dropped by about 5.0.

  • Correct: The acetic acid / acetate mixture resists a pH change: Right: the weak acid/conjugate base pair absorbed the H₃O⁺.
  • The three solutions resist a change in pH to the same extent: The pH changes differ by a factor of about 50.
  • NaCl solution is a buffer because it contains ions: Na⁺ and Cl⁻ do not react with H₃O⁺, so NaCl solution behaves like water.
  • Water resists a change in pH better than the mixture: Water changed by about 5 pH units, far more than the buffer.

2. Which equation shows how the mixture removes the added acid?

  1. H₃O⁺ + CH₃COO⁻ → CH₃COOH + H₂O
  2. H₃O⁺ + OH⁻ → H₂O + H₂O (with OH⁻ from water)
  3. H₃O⁺ + Cl⁻ → HCl + H₂O
  4. CH₃COOH + H₂O → CH₃COO⁻ + H₃O⁺
Show the answer

The added H₃O⁺ is taken up by the base of the pair, acetate, which becomes acetic acid.

  • Correct: H₃O⁺ + CH₃COO⁻ → CH₃COOH + H₂O: Right: the conjugate base consumes added acid.
  • H₃O⁺ + OH⁻ → H₂O + H₂O (with OH⁻ from water): There is very little OH⁻ in an acidic buffer; the acetate does the work.
  • H₃O⁺ + Cl⁻ → HCl + H₂O: Cl⁻, the conjugate base of a strong acid, does not take protons.
  • CH₃COOH + H₂O → CH₃COO⁻ + H₃O⁺: This is the ionization of acetic acid, which would add H₃O⁺, not remove it.

3. Show how the "pH after" for the mixture was calculated: find the pH after the HCl is added.

Type a number.

Show the answer

Before: 0.0100 mol CH₃COOH and 0.0100 mol CH₃COO⁻. After H₃O⁺ + CH₃COO⁻ → CH₃COOH: 0.0110 mol acid, 0.0090 mol acetate. Ka = 10^−4.74 = 1.82 × 10⁻⁵. [H₃O⁺] = Ka × 0.0110/0.0090 = 2.22 × 10⁻⁵ M; pH = 4.65.

  • Answer: 4.65

Particle view

A buffer before and after an addition

Box 1 (before)HAHAHAHAHAA⁻A⁻A⁻A⁻A⁻Box 2HAHAHAHAHAHAHAA⁻A⁻A⁻Box 3HAHAHAA⁻A⁻A⁻A⁻A⁻A⁻A⁻Box 4HAHAHAHAHAA⁻A⁻A⁻A⁻A⁻H₃O⁺H₃O⁺

Key: HA weak acid; A⁻ conjugate base; H₃O⁺ hydronium. Spectator ions and water are not drawn.

4. Two OH⁻ ions are added to Box 1. Which box shows the result?

  1. Box 3
  2. Box 2
  3. Box 4
  4. Box 1, unchanged
Show the answer

OH⁻ + HA → A⁻ + H₂O: two HA become two A⁻, giving 3 HA and 7 A⁻.

  • Correct: Box 3: Right: base removes HA and makes A⁻.
  • Box 2: Box 2 is what two added H₃O⁺ ions would produce.
  • Box 4: Box 4 shows free H₃O⁺ that has not reacted, which a buffer does not allow.
  • Box 1, unchanged: The particles change: the buffer works by reacting with what is added.

5. A student adds two H₃O⁺ ions to Box 1 and draws Box 4 as the result. What is wrong with the drawing?

  1. The H₃O⁺ should have reacted with A⁻, giving 7 HA and 3 A⁻
  2. The H₃O⁺ should have reacted with HA, giving 3 HA and 7 A⁻
  3. Nothing is wrong: added ions stay free in a buffer
  4. The H₃O⁺ should have become two OH⁻ ions
Show the answer

Added H₃O⁺ reacts with A⁻ essentially to completion (H₃O⁺ + A⁻ → HA + H₂O), so it cannot sit beside 5 unreacted A⁻ ions. The correct drawing is Box 2.

  • Correct: The H₃O⁺ should have reacted with A⁻, giving 7 HA and 3 A⁻: Right: the conjugate base consumes the added acid.
  • The H₃O⁺ should have reacted with HA, giving 3 HA and 7 A⁻: H₃O⁺ is an acid; it reacts with the base of the pair, A⁻, not with HA.
  • Nothing is wrong: added ions stay free in a buffer: If added ions stayed free, the mixture would not be a buffer.
  • The H₃O⁺ should have become two OH⁻ ions: An acid does not turn into a base; it gives its proton to A⁻.

6. The weak acid has pKa 4.74. What is the pH of the solution shown in Box 2?

Type a number.

Show the answer

Ka = 10^−4.74 = 1.82 × 10⁻⁵. [H₃O⁺] = Ka × [HA]/[A⁻] = 1.82 × 10⁻⁵ × 7/3 = 4.25 × 10⁻⁵ M; pH = 4.37. The volume cancels, so particle counts can be used directly.

  • Answer: 4.37

7. Box 1 is diluted with water to twice its volume. How does its pH change?

  1. It stays about the same, because the ratio [A⁻]/[HA] is unchanged
  2. It rises by about 0.30, because the concentrations are halved
  3. It falls, because water is added
  4. It rises to 7, because the buffer is diluted
Show the answer

Both [HA] and [A⁻] are halved, so their ratio stays 1 and [H₃O⁺] = Ka × 1. The pH depends on the ratio, not on the absolute concentrations.

  • Correct: It stays about the same, because the ratio [A⁻]/[HA] is unchanged: Right: dilution leaves the ratio alone.
  • It rises by about 0.30, because the concentrations are halved: Halving both concentrations cancels in the ratio; the pH does not shift by log 2.
  • It falls, because water is added: Water does not add H₃O⁺ to a buffer in any amount that matters.
  • It rises to 7, because the buffer is diluted: A diluted buffer keeps its pH until it is so dilute that water's own ions matter.

Part 9 · Summary

Summary

A buffer resists pH change because its two components react with whatever is added: the conjugate base takes up added acid and the weak acid takes up added base. Only the ratio [A⁻]/[HA] sets the pH, so additions shift it slightly and dilution barely at all.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections