Question

In: Chemistry

A certain weak acid, HA, has a Ka value of 2.0×10−7. Part A: Calculate the percent...

A certain weak acid, HA, has a Ka value of 2.0×10−7.

Part A: Calculate the percent ionization of HA in a 0.10 M solution.

Part B: Calculate the percent ionization of HA in a 0.010 M solution.

Solutions

Expert Solution

Part A :

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

initial conc.            c               0         0

change                -ca            +ca      +ca

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

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

                                         = c a2 / (1-a)

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 = 2.0x10-7

          c = concentration = 0.10M

Plug the values we get a = 1.414x10-3

∴ % dissociation =  1.414x10-3 x 100 =  1.414x10-1

Part B :

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

initial conc.            c               0         0

change                -ca            +ca      +ca

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

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

                                         = c a2 / (1-a)

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 = 2.0x10-7

          c = concentration = 0.010M

Plug the values we get a = 4.472x10-3

∴ % dissociation = 4.472x10-3 x 100 = 4.472x10-1


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