Plugging these new values into Henderson-Hasselbalch gives: In the process, the 0.066 moles of F - is reduced:Ġ.066 initial moles F - 0.010 moles reacted with H 3O + = 0.056 moles F - remainingĪlso during this process, more HF is formed by the reaction:Ġ.10 initial moles HF + 0.010 moles from reaction of F - with H 3O + = 0.11 moles HF after reaction However, we are adding the H 3O + to a solution that has F - in it, so the H 3O + will all be consumed by reaction with F. Now, if we add 0.01 moles of HCl to 100 mL of pure water, we would expect the pH of the resulting solution to be 1.00 (0.01 moles/0.10 L = 0.1 M pH = -log(0.1) = 1.0).
For the purposes of this example, we'll let the added H 3O + be equal to 0.01 moles (from 0.01 moles of HCl). However, for our example, let's say that the amount of added H 3O + is smaller than the amount of F - present, so our buffer capacity is NOT exceeded. In this case, the capacity of the buffer will have been exceeded - a situation one tries to avoid. If the F - is used up before reacting away all of the H 3O +, then the remaining H 3O + will affect the pH directly. \įor every mole of H 3O + added, an equivalent amount of the conjugate base (in this case, F -) will also react, and the equilibrium constant for the reaction is large, so the reaction will continue until one or the other is essentially used up. Therefore, if we obtain HF in an aqueous solution, we establish the following equilibrium with only slight dissociation (K a(HF) = 6.6x10 -4, strongly favors reactants): Hydrofluoric acid is a weak acid due to the strong attraction between the relatively small F - ion and solvated protons (H 3O +), which does not allow it to dissociate completely in water. To clarify this effect, we can consider the simple example of a Hydrofluoric Acid (HF) and Sodium Fluoride (NaF) buffer. A buffer is able to resist pH change because the two components (conjugate acid and conjugate base) are both present in appreciable amounts at equilibrium and are able to neutralize small amounts of other acids and bases (in the form of H 3O + and OH -) when the are added to the solution.
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