Strange Buffer Question - any ideas? Watch

BeckaH2
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Hi,

I need to compare the following buffer solutions:
- 25cm3 of 0.1moldm-3 ethanoic acid and 25cm3 of 0.1moldm-3 sodium ethanoate (equimolar concentrations).
- 25cm3 of 0.01moldm-3 ethanoic acid and 25cm3 of 0.1moldm-3 sodium ethanoate (NOT equimolar concentrations).

The pH of the equimolar buffer was 4.48, the pH of the non-equimolar buffer was 5.63.

When 1cm3 or 1moldm-3 HCl was added to both buffers, the equimolar buffer pH changed to 4.47. In the non-equimolar buffer, the pH changed more significantly to 5.45.

I need to explain the chemistry behind why the non-equimolar buffer was less effective (in terms of how the position of equilibrium changed etc.)

Any ideas?
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username986184
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(Original post by BeckaH2)
Hi,

I need to compare the following buffer solutions:
- 25cm3 of 0.1moldm-3 ethanoic acid and 25cm3 of 0.1moldm-3 sodium ethanoate (equimolar concentrations).
- 25cm3 of 0.01moldm-3 ethanoic acid and 25cm3 of 0.1moldm-3 sodium ethanoate (NOT equimolar concentrations).

The pH of the equimolar buffer was 4.48, the pH of the non-equimolar buffer was 5.63.

When 1cm3 or 1moldm-3 HCl was added to both buffers, the equimolar buffer pH changed to 4.47. In the non-equimolar buffer, the pH changed more significantly to 5.45.

I need to explain the chemistry behind why the non-equimolar buffer was less effective (in terms of how the position of equilibrium changed etc.)

Any ideas?
This is an interesting question. The bottom line is buffering capacity is greatest when the concentration of HA and A- is the same, as in the first scenario. With the second scenario there is much more A- than HA and the pH shifts to a greater extent.

Using the numbers given for scenario 1 the ratio of A-/HA goes from 1 to 0.96. For scenario 2 the ratio of A-/HA goes from 10 to 8.17, a much greater change.
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BeckaH2
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I agree with this "With the second scenario there is much more A- than HA and the pH shifts to a greater extent." but surely if HA is limited and A- is in excess, then adding HCl would actually sort of counter the lack of HA? Why is the system less able to respond than an equimolar?
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username986184
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(Original post by BeckaH2)
I agree with this "With the second scenario there is much more A- than HA and the pH shifts to a greater extent." but surely if HA is limited and A- is in excess, then adding HCl would actually sort of counter the lack of HA? Why is the system less able to respond than an equimolar?
Here's a good explanation. Stick with it.


http://chemistry.stackexchange.com/q...-e-when-a-ha-1
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