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Prepared by Lawrence Kok

Tutorial on Bronsted Lowry Conjugate acid base pair, acidic and basic buffer calculation.

Strong / Weak Acid and its Conjugate Base 

Strong Acid forms Weak Conjugate Base• Strong acid HCI dissociate completely form Cl−

(weak conjugate base)

• Cl− weak base won't accept H+ to form back HCI

• HCI → H+ + Cl−  ( one way )

Weak Acid form Strong Conjugate Base• CH3COOH weak acid dissociate partially

form CH3COO- (strong conjugate base)

• CH3COO- (strong base) will accept H+ to form back CH3COOH molecule.

• CH3COOH ↔ CH3COO- + H+ (reversible)

Strong / Weak Base and its Conjugate Acid

Strong Base form Weak Conjugate acid

• Strong Base OH− accept H+ form H2O (weak conjugate acid)• H2O (weak acid) won't lose H+ again to form OH−

• OH−  → H2O (1 way)

Weak Base form Strong Conjugate Acid

• NH3 weak base dissociate partially form NH4+ (strong conjugate acid)

• NH4+ (strong acid) lose H+ to form back NH3 molecule

• NH3 + H2O ↔ NH4+ +OH− (reversible)

Acidic vs Basic Buffer

Acidic Buffer • Contain weak acid and its salt

• CH3COOH (weak acid) + CH3COONa (salt)

• Buffer need weak acid CH3COOH and base CH3COO- to neutralise added H+ or OH−

• CH3COOH ↔ CH3COO- +  H+ ionise partially producing H+ to neutralise OH−

• CH3COONa → CH3COO-  + Na+ ionise fully producing CH3COO- to neutralise H+

• Effective buffer will have equal amt of weak acid CH3COOH and base CH3COO-

Basic BufferContain weak base and its salt• NH3 (weak base) + NH4CI (salt)

• NH3 (base) and NH4+ (acid) will neutralise added H+ or OH−

• NH3 + H2O ↔ NH4+  +  OH−, ionise partially producing NH3 molecule to neutralize

added H+

• NH4CI → NH4+ + CI−, ionise completely producing NH4

+ to neutralise OH−

 

Click HERE for more info

How acidic Buffer works

Contains weak acid and its conjugate salt

Adding of AcidAdding of Base

Weak acid neutralize added base

Conjugate base neutralize added acid

How basic Buffer works

Contains weak base and its conjugate salt

Adding of BaseAdding of Acid

Conjugate acid neutralize added base

Weak base neutralize added acid

Formula for Buffers

Two ways to solve buffer calculation• Ka method• Formula method

Two ways to solve buffer calculation• Ka method• Formula method

Acidic Buffer Calculation

Using formula method, example 1 Using Ka method, example 2

Using Ka method

Acidic Buffer Calculation

Using formula and Ka method

Basic Buffer Calculation

Using formula method

Formula method

Ka method

Formula method

Preparation of Acidic / Basic Buffer

1st method using ratio of acid/saltpH buffer depends on pKa and ratio acid/salt

2nd method using titration method

1st method using ratio of acid/salt

Preparation of acidic buffer and basic buffer using ratio method

Steps• Choose pKa or pKb closest to desired pH• Adjust ratio of acid/salt or base/salt according shown above• Fix Conc ratio but CHANGE Vol ratio

Preparation of acidic buffer and basic buffer using titration method

Steps• Acidic buffer, add half amt of base to neutralize acid (excess acid + salt)• Basic buffer, add half amt of acid to neutralize base (excess base + salt)

Sample calculation on basic buffer preparation using titration method

• Using Kb method to find Conc OH-

Steps • Find conc of base and salt left after neutralization

Click here to view buffer composition for acidic and basic buffer

Click here to view salt hydrolysis

Videos on salt hydrolysis and buffer composition

Acknowledgements

Thanks to source of pictures and video used in this presentation

Thanks to Rich Thornley for his videos

Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/

Prepared by Lawrence Kok

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