Buffer Examples: 44 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Buffer.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A buffer is a solution that resists large changes in pH when small amounts of acid or base are added.

A buffer acts like a chemical shock absorber for pH.

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How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Buffer starts by identifying the acid/base species, ions produced or transferred, and pH evidence.

Common stuck point: Students often know a formula related to buffer but skip the recognition step: Am I tracking acid/base identity, pH, ions in solution, neutralization, buffer behavior, or salt formation? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I tracking acid/base identity, pH, ions in solution, neutralization, buffer behavior, or salt formation?

Common Mistakes to Watch For

Before you work through the examples, skim the mistake guide so you know which shortcuts and sign errors to avoid.

pH Mistakes

Worked Examples

Example 1

medium

A buffer has

[HA] = 0.40\text{ M}

[A^-] = 0.20\text{ M}

pK_a = 4.74

. Find pH.

Answer

p H \approx 4.44

First step

\log(0.20/0.40) = \log(0.5) = -0.301

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Example 2

medium

Make a buffer at pH 5.04 using

pK_a = 4.74

. What ratio

[A^-]/[HA]

is required?

Example 3

medium

Henderson-Hasselbalch with

pK_a = 4.74

[A^-] = 0.10\text{ M}

[HA] = 0.40\text{ M}

. Find pH.

Example 4

hard

1.0\text{ L}

buffer has

0.30\text{ mol}

acetic acid (

pK_a = 4.74

) and

0.30\text{ mol}

acetate. After adding

0.05\text{ mol}

NaOH, find the new pH.

Example 5

hard

Design a

1.0\text{ L}

acetate buffer at pH 5.04 using

0.10\text{ mol}

acetic acid (

pK_a = 4.74

). How many mol of sodium acetate must you add?

Example 6

hard

A buffer contains

0.40\text{ M}

phosphate as

H_2PO_4^-

(

pK_{a2} = 7.21

) and

0.40\text{ M}

HPO_4^{2-}

. After adding

0.10\text{ mol}

HCl to

1.0\text{ L}

, find the new pH.

Example 7

challenge

Design a

1.0\text{ L}

phosphate buffer at pH 7.40 using

pK_{a2} = 7.21

and a total phosphate concentration of

0.10\text{ M}

. Find

[H_2PO_4^-]

and

[HPO_4^{2-}]

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

What is the defining property of a buffer solution?

Example 2

easy

A buffer is typically made from a weak acid and what other component?

Example 3

easy

Does adding a small amount of strong acid to a buffer cause a large pH drop?

Example 4

easy

Is a solution of pure

HCl

a buffer?

Example 5

easy

Name a common biological buffer that keeps blood near pH 7.4.

The carbonic acid/bicarbonate buffer holds blood near pH 7.4.

Example 6

easy

In a buffer, what neutralizes added base (

OH^-

Example 7

easy

Does a buffer work equally well at any pH?

Example 8

easy

Which pair forms a buffer: (a) HCl + NaCl, (b)

CH_3COOH

CH_3COONa

Example 9

medium

Use Henderson-Hasselbalch to find pH of a buffer with

[A^-]=[HA]

and

pK_a=4.76

When [A⁻] = [HA], pH equals pKa

Example 10

medium

A buffer has

[HA]=0.20

[A^-]=0.40

pK_a=4.74

. Find pH.

The 2:1 base-to-acid ratio shifts pH above pKa from 4.74 to 5.04.

Example 11

medium

To make a buffer at pH 5.0 using an acid with

pK_a=4.7

, is more acid or more base needed?

Example 12

medium

After adding 0.01 mol OH- to a buffer with 0.10 mol HA and 0.10 mol A-, find the new mole ratio

A^-/HA

Example 13

medium

Which buffer best holds pH 7.2: acetate (

pK_a=4.7

), phosphate (

pK_a=7.2

), or ammonia (

pK_a=9.2

The best buffer for pH 7.2 is the one whose pKa is closest to 7.2 — phosphate.

Example 14

medium

Why does diluting a buffer with water barely change its pH?

Example 15

medium

A buffer is

0.30

NH_3

and

0.30

NH_4^+

. Given

pK_a(NH_4^+)=9.25

, find pH.

Equal NH₃ and NH₄⁺ concentrations give pH = pKa

Example 16

medium

A buffer has

[HA]=0.50

M and

[A^-]=0.25

M with

pK_a=4.74

. Find the pH.

With more HA than A⁻, the buffer pH of 4.44 falls below pKa 4.74.

Example 17

medium

What is the buffer made from

NH_3

and

NH_4Cl

called, and which species neutralizes added acid?

Example 18

challenge

A 1.0 L buffer has 0.50 mol

HA

and 0.50 mol

A^-

(

pK_a=4.74

). Find the pH after adding 0.10 mol HCl.

Example 19

challenge

How many moles of

NaA

must be added to 1.0 L of 0.20 M

HA

(

pK_a=4.70

) to reach pH 5.00? (Assume volume constant.)

Example 20

challenge

Compare buffer capacity of (a) 0.50 M HA / 0.50 M A- and (b) 0.05 M HA / 0.05 M A-, same pKa. Which resists pH change more and why?

Example 21

easy

Which pair forms a buffer: (a)

HNO_3 + NaNO_3

, (b)

HF + NaF

Example 22

easy

Use Henderson-Hasselbalch:

[A^-] = 0.10\text{ M}

[HA] = 0.10\text{ M}

pK_a = 3.50

. Find pH.

Example 23

easy

A buffer has

pK_a = 6.00

[A^-] = 0.30

[HA] = 0.030

M. Find pH.

Example 24

easy

Which buffer best holds pH 4.7: acetate (

pK_a=4.74

), phosphate (

pK_a=7.21

), or ammonia (

pK_a=9.25

Example 25

medium

[A^-]/[HA] = 2

pK_a = 7.20

. Find pH.

Example 26

medium

1.0\text{ L}

buffer holds

0.20\text{ mol}

HA

and

0.20\text{ mol}

A^-

(

pK_a = 4.74

). After adding

0.05\text{ mol}

NaOH, find new mole ratio

A^-/HA

Example 27

medium

Pick the best buffer system for pH 9.0 from: acetate (

pK_a=4.74

), bicarbonate (

pK_a=6.35

), ammonia (

pK_a=9.25

), phosphate (

pK_a=12.4

Example 28

medium

A buffer at pH =

pK_a

is diluted with an equal volume of water. What is the new pH?

Example 29

medium

What is the conjugate base in the

H_2CO_3 / HCO_3^-

buffer system?

Example 30

medium

A buffer has

[NH_3] = 0.40\text{ M}

[NH_4^+] = 0.20\text{ M}

pK_a(NH_4^+) = 9.25

. Find pH.

Example 31

medium

A buffer has

[A^-] = 0.50\text{ M}

[HA] = 0.50\text{ M}

pK_a = 7.20

. Adding

0.10

H^+

1\text{ L}

— find the new mole ratio.

Example 32

medium

A formic-acid buffer has

pK_a = 3.75

[A^-] = [HA] = 0.20\text{ M}

. Find pH.

Example 33

hard

0.50\text{ L}

buffer has

0.10\text{ mol}

NH_3

and

0.20\text{ mol}

NH_4^+

(

pK_a = 9.25

). Find pH.

Example 34

hard

Adding

0.10\text{ mol}

HCl to

1.0\text{ L}

pure water (initial pH 7) drops pH to about 1.0. The same

0.10\text{ mol}

HCl in a

1.0\text{ L}

buffer (

0.50

M HA /

0.50

A^-

pK_a = 4.74

) gives what new pH?

Example 35

hard

How many moles of NaOH must be added to

1.0\text{ L}

0.50

HA

(

pK_a = 4.74

) to reach pH 4.74?

Example 36

hard

pK_a = 4.74

. To make a buffer at pH 5.34, what ratio

[A^-]/[HA]

is required?

Example 37

hard

1.0\text{ L}

buffer has

0.20\text{ mol}

HA

(

pK_a = 4.74

) and

0.20\text{ mol}

A^-

. After adding

0.30\text{ mol}

HCl, is the buffer still functioning, and what is the approximate pH?

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Related Concepts

Acid Base Chemical Equilibrium Titration

Background Knowledge

These ideas may be useful before you work through the harder examples.

acidbaseequilibrium