Inorganic and Applied Chemistry

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Inorganic and Applied Chemistry

Example 5- C:

pH in the solution with two weak acids

We wish to determine pH in a solution containing 1.00 M HCN and 5.00 M HNO 2. From a table we have

found that the Ka value for HCN is 6.2 · 10-10M while the Ka value is 4.0 · 10-4 M for HNO 2. as both acids

are weak acids, and thereby only partly dissociates, the primarily species in the solution are:

HCN, HNO 2 and H 2 O

The following primary reactions may be written as:

HCN(aq)  H+(aq) + CN-(aq)

HNO 2 (aq)  H+(aq) + NO 2 - (aq)

Such a mixture of two weak acids seems as a complex problem. As nevertheless HNO 2 from the Ka values

is a far stronger acid than HCN (these may be directly compared as the Ka’ s have a similar unit), we will

assume that this acid is the dominating contributor to H+ ions in the solution. We will thereby focus only

on this equilibrium with corresponding equilibrium expression:

HNO 2 (aq)  H+(aq) + NO 2 - (aq)

 

 2

4. 0104 2

HNO

H NO

Ka M

^

We now look at initial and equilibrium conditions similar to other examples. The initial concentrations are:

[HNO 2 ] 0 = 5.00 M

[NO 2 - ] 0 = 0 M

[H+] 0 = 10-7 M (from autoprotolysis of water)

and the end-concentrations are thereby:

[HNO 2 ] 0 = (5.00 – x) M

[NO 2 - ] 0 = x M

[H+] 0 = (10-7 + x) M

which by insertion in the expression for Ka gives:

x M

x

x x

Ka M^2

4 7

4. 510

5. 00

( 10 )

4. 010

(



The concentration of H+ ions is thereby 4.5 · 10-2 M which gives a pH value of:

Acids and bases