Question

For each of the following compounds, indicate the pH at which 50% of the compound will be in a form that possesses a charge and at which pH more than 99% of the compound will be in a form that possesses a charge.
CH3COOH (pKa = 4.76)
CH3NH+3 (pKa = 10.7)

Determine a pH at which pH more than 99% of CH3COOH will be in a form that possesses a charge. Express your answer using two decimal places.

Answer 1

For the weak acid HA the dissociation is written as

HA <-----------> H+ + A-

When the system contains both HA and A- , it acts as a buffer.

Thus the pH of the buffer is given by Hendersen equation as

pH = pKa + log [A-]/[HA]

1) for acetic acid CH3COOH (pKa = 4.76)

i) pH at which 50% charged form (A-) and 50% unchared form (HA) are present

   HA <-----------> H+ + A-

100 0 0 initial

50 50 50 at the given condition

That is [A-]= 50 and [HA ] = 50

thus pH = 4.76 +log 50/50

= 4.76

ii) pH at which 99% is ionised

then

HA <-----------> H+ + A-

100 0 0 initial

1 99 99 at the given condition

Thus pH = 4.76 + log 99/1

=6.75

2) For CH3NH3+ (pKa =10.7)

i) pH at which 50 % is charged

HA <-----------> H+ + A-

100 0 0 initial

50 50 50 at the given condition

pH = 10.7 + log 50/50

= 10.7

ii) pH at which 99% is charged

  

HA <-----------> H+ + A-

100 0 0 initial

99 1 1at the given condition

pH = 10.7 + log 1/99

= 8.71