MYTUTOR SUBJECT ANSWERS

146 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...

4 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

77 SUBJECT SPECIALISTS

£20 /hr

William D.

Degree: Mechanical Engineering (Masters) - Leeds University

Subjects offered: Chemistry, Physics+ 2 more

Chemistry
Physics
Maths
Italian

“I am a British student currently in the first year of a masters degree in Mechanical Engineering at the University of Leeds. Having previously lived in Spain and Italy, I am fluent in both languages and have excellent communications s...”

£20 /hr

Ollie F.

Degree: Biochemistry (Bachelors) - Bristol University

Subjects offered: Chemistry, Science+ 5 more

Chemistry
Science
Physics
Maths
Extended Project Qualification
Biology
-Personal Statements-

“A little bit about me: I'm a Biochemistry student at Bristol University. I've always been facinated with how the world world works both on a smaller and larger scale. The human body has always been an interest of mine, in particular h...”

£26 /hr

Vandan P.

Degree: Physical Natural Sciences (Bachelors) - Cambridge University

Subjects offered: Chemistry, Science+ 6 more

Chemistry
Science
Physics
Maths
Further Mathematics
.STEP.
.MAT.
-Oxbridge Preparation-

“I am a second year undergraduate at Corpus Christi College Cambridge. I am studying Natural Sciences. In my first year I did Physics, Chemistry, Computer Science and Maths and passed with first class honours....”

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

What are amino acids?

What are the Key features of benzene?

what is a simple test to detect the presence of a carbonyl and to identify it?

How can crude oil be used as a source of hydrocarbons?

View A Level Chemistry tutors

Cookies:

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

mtw:mercury1:status:ok