Inorganic and Applied Chemistry

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Inorganic and Applied Chemistry

Example 5- H:

Buffer capacity

We wish to calculate the change in pH when 0.01 moles HCl on gas form is added to 1.0 L of a solution

consisting of 5.00 M CH 3 COOH and 5.00 M CH 3 COONa.

Acetic acid has a pKa value of 4.74. In this case [CH 3 COOH] = [CH 3 COO-] before HCl is added which is

why pH initially may be found via the buffer equation:

 



log 4. 74 log( 1 ) 4. 74

3

3

CHCOO^

pH pK CHCOOH

a

After the addition of HCl the important components in respect to pH are:

CH 3 COOH, Na+, CH 3 COO-, H 3 O+, Cl- and H 2 O

Due to the addition of HCl there is quite an amount of H 3 O+ present in the solution which is why the most

important reaction is:

H+(aq) + CH 3 COO-(aq)  CH 3 COOH (aq)

We assume that the reaction runs completely as H 3 O+ is a strong acid. We will look at the initial and

equilibrium conditions analogously prior examples. The initial concentrations are:

[CH 3 COOH] 0 = 5.00 M

[CH 3 COO-] 0 = 5.00 M

[H 3 O+] 0 = 0.01 M (the autoprotolysis of water is neglected)

and the final concentrations are thereby:

[CH 3 COOH] = 5.01 M

[CH 3 COO-] = 4.99 M

[H 3 O+] = 0 M

We still have a buffer system as the corresponding weak acid-base pair is present in amounts of the same

magnitude. We may therefore calculate pH using the buffer equation:





4. 74

4. 99

log 4. 74 log^5.^01

3

3 *

CHCOO^

pH pK CHCOOH

a

This will in other words mean that the pH of the solution more or less has not changed even though HCl

has been added. This clearly shows the damping capabilities of the buffer system.

Acids and bases