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Lecture 21

# Lecture 21 - Lecture 21 Buffers A buffer is a solution of...

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Lecture 21: Buffers A buffer is a solution of an acid and its conjugate base, where the addition of a small additional amount of either will only have a negligible effect on the pH. This occurs when the initial concentrations of both the acid and conjugate base are >10%. Recall the essential equation: [H+][B]/[HB+] = Ka. In the problems that we have done so far, the initial solution consisted of ONE of the species HB+ or B added to water. Also, one of the species is assumed to be in much larger concentration at equilibrium than the other one. In this situation, further additions of either HB+ or B will change the pH substantially. Let’s say that a large amount of weak acid HB+ is present. We have been assuming that [H+] and [B] are small and equal. Obviously, adding still more HB+ will increase [H+], because Ka has to stay constant. If we add even a small further amount of B, [H+] will decrease considerably. Now, let’s consider a different case, that of buffers. In a buffer solution, significant amounts of both B and HB+ are added to form the initial solution.

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