Henderson-Hasselbalch Equation

[Dr Gerrard]

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All right, I am having trouble conceptualizing this.

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

So, lets say pH of a particular solution is 4, pKa is 5.

so -1 = log [A-]/[HA]

1/10 = [A-]/[HA]

If the pH is lower than the pKa, why is there a greater concentration in the acid form rather than the conjugate base form?

Since the pH is lower, wouldn't there be a greater concentration of hydrogen ions and thus a greater concentration of conjugate base and a lower concentration of not dissociated acid?

 

[loveoforganic]

Picture this. You have a 10 mL of anhydrous sulfuric acid, and you add 1 mL of water to it (don't do this). You now have a highly concentrated acidic solution. Let's say pH is -2. You also have 1 mL of anhydrous acetic acid, pKa 4.75. If you added the acetic acid to the solution, would you expect that the acetic acid would deprotonate? The pH is lower than the pKa.

 

[Dr Gerrard]

I can't believe I didn't get that. Thanks.