Unit 8 · Topic 8.4 Beta

Acid-Base Reactions and Buffers

When a strong acid or base meets a weak one, the reaction goes to completion, so you react the moles first and then ask what is left.

Practice 1: Models and RepresentationsPractice 5: Mathematical RoutinesPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Your blood stays between pH 7.35 and 7.45 even after a sprint floods it with lactic acid. Pure water would swing by several pH units with the same acid. The difference is a buffer: a weak acid and its conjugate base side by side, each ready to soak up whatever is added.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What is the conjugate base of H₂CO₃?

  1. HCO₃⁻
  2. CO₃²⁻
  3. H₃CO₃⁺
  4. CO₂
Show the answer

Remove one proton: H₂CO₃ → HCO₃⁻.

  • Correct: HCO₃⁻:
  • CO₃²⁻:
  • H₃CO₃⁺:
  • CO₂:

2. Ka of HF is 6.8 × 10⁻⁴. What is Kb of F⁻?

  1. 1.5 × 10⁻¹¹
  2. 6.8 × 10⁻⁴
  3. 1.5 × 10³
  4. 6.8 × 10⁻¹⁸
Show the answer

Kb = Kw / Ka = 1.0 × 10⁻¹⁴ / 6.8 × 10⁻⁴ = 1.5 × 10⁻¹¹.

  • Correct: 1.5 × 10⁻¹¹:
  • 6.8 × 10⁻⁴:
  • 1.5 × 10³:
  • 6.8 × 10⁻¹⁸:

3. 30.0 mL of 0.100 M NaOH is added to 20.0 mL of 0.100 M HCl. What is left over?

  1. 1.00 × 10⁻³ mol OH⁻
  2. 1.00 × 10⁻³ mol H₃O⁺
  3. Nothing: the solution is neutral
  4. 5.00 × 10⁻³ mol OH⁻
Show the answer

3.00 × 10⁻³ mol OH⁻ − 2.00 × 10⁻³ mol H₃O⁺ = 1.00 × 10⁻³ mol OH⁻ in excess.

  • Correct: 1.00 × 10⁻³ mol OH⁻:
  • 1.00 × 10⁻³ mol H₃O⁺:
  • Nothing: the solution is neutral:
  • 5.00 × 10⁻³ mol OH⁻:

Part 4 · See it

See it first

A buffer box holds similar numbers of HA molecules and A minus ions. Added strong acid reacts with A minus to make HA; added strong base reacts with HA to make A minus. Either way the added ion is used up and the ratio of A minus to HA changes only a little, so the pH changes only a little.
A buffer holds both members of a conjugate pair. Each takes care of one kind of addition. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. A strong acid or base reacting with a weak partner has a huge Kthe reaction goes essentially to completion: do it first, with moles
  2. Partly neutralizing a weak acid leaves both HA and A⁻the mixture is a buffer, and [H₃O⁺] = Ka × [HA]/[A⁻]
  3. Fully neutralizing a weak acid leaves only its conjugate basethe solution is basic at equivalence, not neutral
  4. Added H₃O⁺ meets A⁻ and added OH⁻ meets HA in a bufferthe added ions are used up and the pH changes only a little

Part 6 · Key ideas

Key ideas

  • Strong + weak acid-base reactions go to completion (K = Ka/Kw or Kb/Kw, huge). Use moles, react first, then find the pH.
  • At equivalence: strong acid + strong base gives pH 7; weak acid + strong base gives a basic solution; weak base + strong acid an acidic one.
  • A buffer is a weak acid with its conjugate base (or a weak base with its conjugate acid) in similar amounts.
  • Make a buffer by mixing HA with a salt of A⁻, or by adding strong base to a weak acid (less than one mole of base per mole of acid).

Part 7 · Misconception

A common mistake

The wrong idea: Equal moles of acid and base always give a neutral solution.

What actually happens: Only for a strong acid with a strong base. A weak acid neutralized by a strong base leaves its conjugate base, which makes the solution basic; a weak base with a strong acid leaves an acidic solution.

Part 8 · Check yourself

Check yourself

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

Data table

Four mixtures of acetic acid and sodium hydroxide

Each flask starts with 50.0 mL of 0.100 M acetic acid, CH₃COOH (Ka = 1.8 × 10⁻⁵). A different volume of 0.100 M NaOH is added to each. Temperature 25 °C.

Volume of NaOH added
FlaskVolume of 0.100 M NaOH added (mL)
10.0
225.0
350.0
460.0

1. Which flask contains a buffer?

  1. Flask 2
  2. Flask 1
  3. Flask 3
  4. Flask 4
Show the answer

Flask 2 has 5.00 × 10⁻³ mol acetic acid and 2.50 × 10⁻³ mol NaOH. The OH⁻ converts half the acid to acetate, leaving equal amounts of CH₃COOH and CH₃COO⁻: a weak acid with its conjugate base.

  • Correct: Flask 2: Right: weak acid and conjugate base in similar amounts.
  • Flask 1: Flask 1 holds only acetic acid; there is no conjugate base to react with added acid.
  • Flask 3: Flask 3 is exactly neutralized: all the acid became acetate, so there is no weak acid left.
  • Flask 4: Flask 4 has excess NaOH and no acetic acid left; its pH is set by the strong base.

2. What is the pH in Flask 2?

Type a number.

Show the answer

After reaction: 2.50 × 10⁻³ mol CH₃COOH and 2.50 × 10⁻³ mol CH₃COO⁻. In Ka = [H₃O⁺][A⁻]/[HA], the volume cancels, so [H₃O⁺] = Ka × (2.50 × 10⁻³)/(2.50 × 10⁻³) = 1.8 × 10⁻⁵ M and pH = −log(1.8 × 10⁻⁵) = 4.74, which is pKa.

  • Answer: 4.74

3. What is the pH in Flask 3?

Type a number.

Show the answer

All 5.00 × 10⁻³ mol acid became acetate, now in 100.0 mL: [CH₃COO⁻] = 0.0500 M. Kb = Kw/Ka = 5.56 × 10⁻¹⁰. [OH⁻] = √(5.56 × 10⁻¹⁰ × 0.0500) = 5.27 × 10⁻⁶ M; pOH = 5.278; pH = 8.72.

  • Answer: 8.72

4. What is the pH in Flask 4?

Type a number.

Show the answer

OH⁻ added: 6.00 × 10⁻³ mol; acid: 5.00 × 10⁻³ mol. Excess OH⁻ = 1.00 × 10⁻³ mol in 0.1100 L = 0.00909 M. The acetate adds very little OH⁻ beside this. pOH = 2.041; pH = 11.96.

  • Answer: 11.96

Particle view

Four solutions

PHFHFHFHFF⁻F⁻F⁻F⁻Na⁺Na⁺Na⁺Na⁺QH₃O⁺H₃O⁺H₃O⁺H₃O⁺Cl⁻Cl⁻Cl⁻Cl⁻Cl⁻Cl⁻Na⁺Na⁺RF⁻F⁻F⁻F⁻F⁻F⁻Na⁺Na⁺Na⁺Na⁺Na⁺Na⁺SHFHFHFHFHFHF

Key: HF hydrofluoric acid molecule; F⁻ fluoride; H₃O⁺ hydronium; Na⁺ sodium ion; Cl⁻ chloride. Water is not drawn, nor the few H₃O⁺ and F⁻ ions made when HF ionizes.

5. Which box represents a buffer?

  1. P
  2. Q
  3. R
  4. S
Show the answer

Box P has a weak acid (HF) and its conjugate base (F⁻) in equal amounts. Added H₃O⁺ reacts with F⁻; added OH⁻ reacts with HF.

  • Correct: P: Right: weak acid plus conjugate base.
  • Q: Box Q is a strong acid solution with spare Cl⁻. Cl⁻ is too weak a base to remove added H₃O⁺.
  • R: Box R has the base F⁻ but no HF to react with added OH⁻.
  • S: Box S has the weak acid but no F⁻ to react with added H₃O⁺.

6. Box Q contains Cl⁻, the conjugate base of HCl. Why is it not a buffer?

  1. Cl⁻ is the conjugate base of a strong acid, so it does not take H⁺ from added H₃O⁺
  2. It has more Cl⁻ than H₃O⁺, and a buffer needs equal amounts
  3. Na⁺ ions react with added OH⁻ and use it up
  4. A buffer is a solution with no H₃O⁺ ions
Show the answer

HCl ionizes fully, so its conjugate base has a negligible Kb: Cl⁻ does not pick protons back up. Added acid would stay as free H₃O⁺, and the pH would drop sharply.

  • Correct: Cl⁻ is the conjugate base of a strong acid, so it does not take H⁺ from added H₃O⁺: Right: a buffer needs a conjugate base strong enough to react with added acid.
  • It has more Cl⁻ than H₃O⁺, and a buffer needs equal amounts: Buffers work over a range of ratios; unequal amounts are not the problem here.
  • Na⁺ ions react with added OH⁻ and use it up: Na⁺ is a spectator ion; it does not react with OH⁻.
  • A buffer is a solution with no H₃O⁺ ions: Every water solution has some H₃O⁺, buffers included.

Experimental setup

Two acid-base reactions

Reaction I: aqueous HF (Ka = 6.8 × 10⁻⁴) is mixed with aqueous NaOH.

Reaction II: aqueous NH₃ (Kb = 1.8 × 10⁻⁵) is mixed with aqueous HCl.

7. Equal moles of NH₃ and HCl react in Reaction II. Which describes the final solution?

  1. Acidic, because NH₄⁺ gives protons to water
  2. Neutral, because equal moles of acid and base reacted
  3. Basic, because NH₃ is a base
  4. Neutral, because Cl⁻ is a spectator ion
Show the answer

The product NH₄⁺ is the conjugate acid of a weak base: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺ (Ka = Kw/Kb = 5.6 × 10⁻¹⁰), so the solution is slightly acidic.

  • Correct: Acidic, because NH₄⁺ gives protons to water: Right: the conjugate acid of a weak base is a weak acid.
  • Neutral, because equal moles of acid and base reacted: Equal moles give a neutral solution only for strong acid with strong base.
  • Basic, because NH₃ is a base: All the NH₃ was converted to NH₄⁺; no base is left.
  • Neutral, because Cl⁻ is a spectator ion: Cl⁻ is a spectator, but NH₄⁺ is not: it reacts with water.

Part 9 · Summary

Summary

When a strong acid or base meets a weak one, the reaction goes to completion, so you react the moles first and then ask what is left. Leftover weak acid plus its conjugate base is a buffer, with [H₃O⁺] = Ka × [HA]/[A⁻]; only the conjugate base left means a basic solution; excess strong base sets the pH by itself.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections