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How to Make a Buffer Solution for A Level Chemistry - Easy Guide

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sheila

02/01/2023

Chemistry

Chapter 21.1 - Buffer Solutions

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PH and POH of Strong Acids and Bases

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PH, POH, Strong Acids

How to Make a Buffer Solution for A Level Chemistry - Easy Guide

Buffer solutions are essential in maintaining stable pH levels in various chemical and biological systems. This summary explores the concept of buffer solutions, their preparation, and pH calculations in A Level Chemistry. It covers key aspects of how to make a buffer solution, the components of buffer solutions, and methods for calculating the pH of buffer solutions.

  • Buffer solutions minimize pH changes when small amounts of acids or bases are added
  • They consist of a weak acid (HA) and its conjugate base (A-)
  • Two approaches for preparing buffer solutions: mixing weak acid with its salt or partial neutralization
  • pH calculations involve using the acid dissociation constant (Ka) and concentrations of acid and conjugate base
...

02/01/2023

233

21.1-Buffer solutions Key facts: - buffer solution a system that minimises PH Changes when smau amounts of an acid/ alkali are added - buffe

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Preparing Buffer Solutions

This section discusses two approaches to prepare buffer solutions in the lab, which is an essential skill for A Level Chemistry students.

Approach 1: Mixing Solutions

The first method involves mixing a solution of a weak acid with a solution containing the salt of the weak acid.

Example: To create an acetate buffer, you might mix a solution of acetic acid CH3COOHCH3COOH with a solution of sodium acetate CH3COONaCH3COONa.

Approach 2: Weak Neutralization

The second method involves adding an aqueous solution of alkali to an excess of a weak acid.

Highlight: In this approach, the weak acid is partially neutralized, resulting in a solution containing both unreacted HA and the A- salt.

Example: To prepare a phosphate buffer, you could add a calculated amount of sodium hydroxide NaOHNaOH to a solution of phosphoric acid H3PO4H3PO4.

Both methods result in a solution containing a weak acid and its conjugate base, which is the essence of a buffer solution.

Vocabulary: Conjugate base - The species formed when an acid donates a proton. In the case of acetic acid CH3COOHCH3COOH, the conjugate base is the acetate ion CH3COO−CH3COO-.

21.1-Buffer solutions Key facts: - buffer solution a system that minimises PH Changes when smau amounts of an acid/ alkali are added - buffe

View

Calculating pH of Buffer Solutions

Understanding how to calculate the pH of buffer solutions is crucial for A Level Chemistry students, particularly when dealing with buffer calculations in AQA A Level Chemistry questions.

The calculation of buffer pH involves the use of the Henderson-Hasselbalch equation, which is derived from the acid dissociation constant KaKa expression:

Ka = H+H+A−A- / HAHA

Rearranging this equation gives:

H+H+ = Ka × HAHA / A−A-

Highlight: The value of Ka provides the 'coarse tuning' of the buffer's pH, while the ratio of HAHA to A−A- allows for 'fine tuning'.

Key points to remember:

  • The pH of a buffer depends on the Ka value and the ratio of concentrations of HA and A-
  • When HAHA = A−A-, pH = pKa
  • For more precise calculations, use the actual concentrations rather than assuming a 1:1 ratio

Example: For a buffer made from a weak acid with pKa = 4.7, if HAHA = A−A-, the pH would be 4.7.

Vocabulary: pKa - The negative logarithm of the acid dissociation constant KaKa. It's a measure of the strength of an acid in solution.

21.1-Buffer solutions Key facts: - buffer solution a system that minimises PH Changes when smau amounts of an acid/ alkali are added - buffe

View

Practical Example: Calculating Buffer pH

This section provides a practical example of how to calculate the pH of a buffer solution, which is a common type of question in A Level Chemistry buffer calculations.

Example: Ethanoic acid buffer

Given:

  • 125 cm³ of 0.8 mol dm⁻³ ethanoic acid CH3COOHCH3COOH
  • 75 cm³ of 1 mol dm⁻³ sodium ethanoate CH3COONaCH3COONa
  • Ka of ethanoic acid = 1.74 × 10⁻⁵ mol dm⁻³

Step 1: Calculate new concentrations after mixing

CH3COOHCH3COOH = 0.8×1250.8 × 125 / 200 = 0.5 mol dm⁻³ CH3COO−CH3COO⁻ = 1×751 × 75 / 200 = 0.375 mol dm⁻³

Step 2: Use the equation H+H+ = Ka × HAHA / A−A-

H+H+ = 1.74×10−5×0.51.74 × 10⁻⁵ × 0.5 / 0.375 = 2.32 × 10⁻⁵ mol dm⁻³

Step 3: Calculate pH

pH = -log2.32×10−52.32 × 10⁻⁵ = 4.63

Highlight: This example demonstrates how to prepare a buffer solution with a specific pH and how to calculate pH of buffer solution given molarity and volume.

Example: If you needed to prepare a pH 4 buffer solution, you would adjust the ratio of acid to conjugate base to achieve this pH.

Understanding these calculations is crucial for mastering buffers in A Level Chemistry OCR and other exam boards.

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Subjects

Chemistry

Acids and Bases

PH and POH of Strong Acids and Bases

PH, POH, Strong Acids

 

Chemistry

•

233

•

2 Jan 2023

•

4 pages

How to Make a Buffer Solution for A Level Chemistry - Easy Guide

user profile picture

sheila

@study_withsheila

Buffer solutions are essential in maintaining stable pH levels in various chemical and biological systems. This summary explores the concept of buffer solutions, their preparation, and pH calculations in A Level Chemistry. It covers key aspects of how to... Show more

21.1-Buffer solutions Key facts: - buffer solution a system that minimises PH Changes when smau amounts of an acid/ alkali are added - buffe

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Preparing Buffer Solutions

This section discusses two approaches to prepare buffer solutions in the lab, which is an essential skill for A Level Chemistry students.

Approach 1: Mixing Solutions

The first method involves mixing a solution of a weak acid with a solution containing the salt of the weak acid.

Example: To create an acetate buffer, you might mix a solution of acetic acid CH3COOHCH3COOH with a solution of sodium acetate CH3COONaCH3COONa.

Approach 2: Weak Neutralization

The second method involves adding an aqueous solution of alkali to an excess of a weak acid.

Highlight: In this approach, the weak acid is partially neutralized, resulting in a solution containing both unreacted HA and the A- salt.

Example: To prepare a phosphate buffer, you could add a calculated amount of sodium hydroxide NaOHNaOH to a solution of phosphoric acid H3PO4H3PO4.

Both methods result in a solution containing a weak acid and its conjugate base, which is the essence of a buffer solution.

Vocabulary: Conjugate base - The species formed when an acid donates a proton. In the case of acetic acid CH3COOHCH3COOH, the conjugate base is the acetate ion CH3COO−CH3COO-.

21.1-Buffer solutions Key facts: - buffer solution a system that minimises PH Changes when smau amounts of an acid/ alkali are added - buffe

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Calculating pH of Buffer Solutions

Understanding how to calculate the pH of buffer solutions is crucial for A Level Chemistry students, particularly when dealing with buffer calculations in AQA A Level Chemistry questions.

The calculation of buffer pH involves the use of the Henderson-Hasselbalch equation, which is derived from the acid dissociation constant KaKa expression:

Ka = H+H+A−A- / HAHA

Rearranging this equation gives:

H+H+ = Ka × HAHA / A−A-

Highlight: The value of Ka provides the 'coarse tuning' of the buffer's pH, while the ratio of HAHA to A−A- allows for 'fine tuning'.

Key points to remember:

  • The pH of a buffer depends on the Ka value and the ratio of concentrations of HA and A-
  • When HAHA = A−A-, pH = pKa
  • For more precise calculations, use the actual concentrations rather than assuming a 1:1 ratio

Example: For a buffer made from a weak acid with pKa = 4.7, if HAHA = A−A-, the pH would be 4.7.

Vocabulary: pKa - The negative logarithm of the acid dissociation constant KaKa. It's a measure of the strength of an acid in solution.

21.1-Buffer solutions Key facts: - buffer solution a system that minimises PH Changes when smau amounts of an acid/ alkali are added - buffe

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

By signing up you accept Terms of Service and Privacy Policy

Practical Example: Calculating Buffer pH

This section provides a practical example of how to calculate the pH of a buffer solution, which is a common type of question in A Level Chemistry buffer calculations.

Example: Ethanoic acid buffer

Given:

  • 125 cm³ of 0.8 mol dm⁻³ ethanoic acid CH3COOHCH3COOH
  • 75 cm³ of 1 mol dm⁻³ sodium ethanoate CH3COONaCH3COONa
  • Ka of ethanoic acid = 1.74 × 10⁻⁵ mol dm⁻³

Step 1: Calculate new concentrations after mixing

CH3COOHCH3COOH = 0.8×1250.8 × 125 / 200 = 0.5 mol dm⁻³ CH3COO−CH3COO⁻ = 1×751 × 75 / 200 = 0.375 mol dm⁻³

Step 2: Use the equation H+H+ = Ka × HAHA / A−A-

H+H+ = 1.74×10−5×0.51.74 × 10⁻⁵ × 0.5 / 0.375 = 2.32 × 10⁻⁵ mol dm⁻³

Step 3: Calculate pH

pH = -log2.32×10−52.32 × 10⁻⁵ = 4.63

Highlight: This example demonstrates how to prepare a buffer solution with a specific pH and how to calculate pH of buffer solution given molarity and volume.

Example: If you needed to prepare a pH 4 buffer solution, you would adjust the ratio of acid to conjugate base to achieve this pH.

Understanding these calculations is crucial for mastering buffers in A Level Chemistry OCR and other exam boards.

21.1-Buffer solutions Key facts: - buffer solution a system that minimises PH Changes when smau amounts of an acid/ alkali are added - buffe

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Understanding Buffer Solutions

Buffer solutions play a crucial role in maintaining stable pH levels in various chemical and biological systems. This section explores the fundamental concepts of buffer solutions and their mechanisms.

Definition: A buffer solution is a system that minimizes pH changes when small amounts of an acid or alkali are added.

The key components of a buffer solution are:

  1. A weak acid HAHA
  2. Its conjugate base A−A-

Highlight: Buffer solutions require a large reservoir of both HA and A- to effectively resist pH changes.

The mechanism of buffer action can be understood in two scenarios:

  1. When acid is added: H+ ions are introduced to the solution The large reservoir of A- reacts with excess H+ The equilibrium shifts to the left H+H+ and pH are restored
  2. When alkali is added: OH- ions are introduced H+ + OH- → H2O reaction occurs H+H+ decreases HA dissociates The equilibrium shifts to the right H+H+ and pH are restored

Example: In a buffer solution containing acetic acid CH3COOHCH3COOH and acetate ion CH3COO−CH3COO-, adding a small amount of HCl would cause the acetate ions to neutralize the added H+ ions, maintaining the pH.

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Thomas R

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Basil

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This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.

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