Inorganic and Applied Chemistry

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Inorganic and Applied Chemistry

Example 5- F:

pH in a solution of foreign ions

In an earlier example we have found that the concentration of H 3 O+ ions in a 1.0 M solution of HF is

2.6 · 10-2 M by which the degree of dissociation is 2.6 %. We wish to determine pH in a solution

containing 1.0 M HF as well as 1.0 M NaF. The Ka value for HF is still 7.2 · 10-4M. Again it is determined

which of the components that are the determining for the pH of the solution. In this case it is:

HF and F-

We wish to determine the position of the following equilibrium:

HF(aq)  H 3 O+(aq) + F-(aq)

On the basis on the reaction scheme we have the following equilibrium expression:

  

HF

HO F

Ka M

^

    7. 2104 3

We will now looks at the initial and equilibrium conditions analogously to prior examples. The initial

concentrations are:

[HF] 0 = 1.0 M

[F-] 0 = 1.0 M (from the dissolved NaF)

[H 3 O+] 0 = 10-7 M (from the autoprotolysis of water)

and the end concentrations are thereby:

[HF] = (1.0 M – x) M

[F-] = (1.0 + x) M

[H 3 O+] = (10-7 + x) M

Which gives by insertion into the expression for Ka:

x M

x

K M x x

a

4

7

(^47). 2 10

1. 
   


2. 210
   (^10 ) (^1.^0 )(
   ^ 
   Thus the concentrations of H 3 O+ ions are 7.2 · 10-4 M why pH becomes:
   pH log 7. 210
   4  3. 1
   Again the contribution from autoprotolysis of water is added although in may in practice be neglected.
   Acids and bases