MYTUTOR SUBJECT ANSWERS

380 views

How do buffers work?

What is a buffer?

A buffer is a solution that resists a change in pH when small amounts of acid or base are added. It is composed of either a weak acid or a weak base with its respective salt.

What's going on in the buffer?

For clarity, let's use the example ethanoic acid and sodium ethanoate.

CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq) (1)

CH3COO-Na+ (aq) ⇌ CH3COO- (aq) + Na+ (aq) (2)

The acid and its salt partially and reversibly dissociate, creating the above equilibriums.

What is the concentration of the buffer?

[H+] = (Ka[CH3COOH]) / [CH3COO-Na+]

pH = - log([H+])

Note:

[acid] = mol. acid / V

[salt] = mol.salt / V

V = the total volume and so:

[acid] / [salt] = mol. acid / mol. salt

Therefore, [H+] = (Ka(mol. CH3COOH)) / (mol. CH3COO-Na+)

Adding acid

The H+ ions react with the CH3COO- ions.

CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq) (1)

Equilibrium pushed , increasing [CH3COOH].

CH3COO-Na+ (aq) ⇌ CH3COO- (aq) + Na+ (aq) (2)

Equilibrium pushed , decreasing [CH3COO-Na+]

As all stoichiometric ratios are 1:1 and total volume is constant:

[H+] = (Ka(mol. CH3COOH + x)) / (mol. CH3COO-Na+ - x)

where x is the number of moles of acid added. As this is a small number, there is only a negligible change in pH = - log ([H+])

Adding a base

The alkali neutralises the acid.

CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq) (1)

Equilibrium pushed , decreasing [CH3COOH].

CH3COO-Na+ (aq) ⇌ CH3COO- (aq) + Na+ (aq) (2)

Equilibrium pushed , increasing [CH3COO-Na+]

As all stoichiometric ratios are 1:1 and total volume is constant:

[H+] = (Ka(mol. CH3COOH - x)) / (mol. CH3COO-Na+ + x)

where x is the number of moles of acid added. As this is a small number, there is only a negligible change in pH = - log ([H+])

Note:

Ka = acid dissociation constant

[X] = concentration of X

mol. x = moles of x

and log = logarithm base 10

Daisy D. A Level Chemistry tutor, GCSE Chemistry tutor, A Level Maths...

9 months ago

Answered by Daisy, an A Level Chemistry tutor with MyTutor


Still stuck? Get one-to-one help from a personally interviewed subject specialist

115 SUBJECT SPECIALISTS

£22 /hr

Lucy S.

Degree: Chemistry (Masters) - Durham University

Subjects offered:Chemistry, Maths

Chemistry
Maths

“Reading Chemistry at Durham University with expertise in Chemistry and Mathematics (A*A*) at GCSE/A-level”

£24 /hr

Ed W.

Degree: Medicine (Bachelors) - Edinburgh University

Subjects offered:Chemistry, Maths+ 3 more

Chemistry
Maths
Further Mathematics
Biology
-Medical School Preparation-

“Hi! I'm Ed and I'm currently studying Medicine at the University of Edinburgh. I tutor at GCSE and A Level forBiology, Chemistry and Maths, and at GCSE level forFurther Maths. These are subjects I'm very comfortable in and enjoy. With ...”

£36 /hr

Steven A.

Degree: Bioscience (Masters) - Durham University

Subjects offered:Chemistry, Science+ 5 more

Chemistry
Science
Maths
Human Biology
English
-Personal Statements-

“Hey! I'm Steven, 18 years old and I'm studying Biochemistry at Durham University. I love tutoring people, to put it simply. At my college i was a tutor to a student in the year below me for chemistry and growing up with a younger sist...”

About the author

£20 /hr

Daisy D.

Degree: Natural Sciences: Chemistry and Mathematics, MSci (Other) - Durham University

Subjects offered:Chemistry, Physics+ 2 more

Chemistry
Physics
Maths
Further Mathematics

“Hi, I'm Daisy! I'm currently in my 3rd year studying an integrated masters course in Natural Sciences (Chemistry and Maths) at Durham University. My main area of interest is the physcial side of both subjects. From a young age, I hav...”

MyTutor guarantee

You may also like...

Posts by Daisy

A stationary radium atom decays, emiting an alpha particle. Why is the recoil speed of the nucleus small compared to the alpha particle?

How do buffers work?

How do I find the molecule from the 1H NMR spectrum?

How is crude oil separated into fractions?

Other A Level Chemistry questions

In d block chemistry, Copper and Chromium electron configuration do not follow the electron filling trend , why is this?

What is a disproportionation reaction?

What is meant by the term chiral?

Describe the 3 types of isomers?

View A Level Chemistry tutors

We use cookies to improve your site experience. By continuing to use this website, we'll assume that you're OK with this. Dismiss

mtw:mercury1:status:ok