Question

A certain weak acid, HA, has a Ka value of 9.1×10−7. Part A Calculate the percent...

A certain weak acid, HA, has a Ka value of 9.1×10−7.

Part A

Calculate the percent dissociation of HA in a 0.10 M solution.

Part B

Calculate the percent dissociation of HA in a 0.010 M solution.

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Answer #1

Part A :

Let a be the dissociation of the weak acid,HA
                            HA <---> H + + A-

initial conc.            c               0         0

Equb. conc.         c(1-a)          ca       ca

Dissociation constant , Ka = ca x ca / ( c(1-a)

                                         = c a2 / (1-α)

In the case of weak acids α is very small so 1-a is taken as 1

So Ka = ca2

==> a = √ ( Ka / c )

Given Ka = 9.1x10-7

          c = concentration = 0.10 M

Plug the values we get a = 3.02 x10-3

∴ % dissociation = 3.02x10-3 x 100 = 0.3 %

Part B :

a = √ ( Ka / c )

Given Ka = 9.1x10-7

          c = concentration = 0.010 M

Plug the values we get a = 9.54 x10-3

∴ % dissociation = 9.54 x10-3 x 100 = 0.95 %

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