The  pK_{a} of a weak acid \left ( HA \right )is 4,5.The pOH of an aqueous bufferd solution of HA in which 50% of  the acid is ionized is

  • Option 1)

    7.0

  • Option 2)

    4.5

  • Option 3)

    2.5

  • Option 4)

    9.5

 

Answers (1)

As we learnt in 

Buffer Solutions -

The solutions which resist change in pH on dilution or with addition of small amounts of acid or alkali are called buffer solution.  

- wherein

Many body fluids  e.g.  blood

 

 and

 

p(Kw) -

K_{w}=[H_{2}O^{+}]\:[H\bar{O}]=10^{-14}

Although  K_{w}  may change with temperature the variation in pH with temperature are so small that we often ignore it.

- wherein

p(K_{w})=-logK_{w}

               =-log{[H_{3}O^{+}]\:[\bar{O}H]}

               =-log10^{-14}

p(K_{w})=p(H)+p(OH)

                 =14

 

 

For acidic buffer,   pH=pK_{a}+log\, \frac{\left [ A^{-} \right ]}{\left [ HA \right ]}

When the acid is 50% dissociated , \left [ A^{-} \right ]=\left [ HA \right ]

or\; \; \; pH=pK_{a}+log1\; \; \; or\; \; pH=pK_{a}

Given\; \; pK_{a}=4.5\; \; \therefore pH=4.5

\therefore \; \; pOH=14-4.5=9.5


Option 1)

7.0

This option is incorrect

Option 2)

4.5

This option is incorrect

Option 3)

2.5

This option is incorrect

Option 4)

9.5

This option is correct

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