Question

In: Chemistry

determine the [OH-] and pH of a solution tht is 0.140 M in F-.

determine the [OH-] and pH of a solution tht is 0.140 M in F-.

Solutions

Expert Solution

Answer –

We are given, [F-] = 0.140 M

First we need to look the Kb value for F- from the Ka value of HF

The Ka value for HF = 7.2*10-4

So, Kb for F- = 1*10-14 / Ka

                       = 1*10-14 / 7.2*10-4

                      = 1.39*10-11

Now we need to put ICE chart

    F- + H2O -----> HF + OH-

I 0.140                  0         0

C    -x                  +x      +x

C 0.140-x          +x      +x

We know,

Kb = [HF] [OH-] / [F-]

1.39*10-11 = x*x/(0.140-x)

We can neglect x in the 0.140-x, because Kb value is too small

x2 = 1.39*10-11 *0.140

      = 1.92*10-12

x = 1.39*10-6 M

so, x = [OH-] = 1.39*10-6 M

now we can calculate pOH from [OH-]

pOH = -log [OH-]

        = - log 1.39*10-6 M

        = 5.86

So, pH = 14- pOH

            = 14 -5.86

            = 8.14


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