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Solving acid base problems

  • 03.06.2019
Solving acid base problems

For example, suppose the concentrations of the acid and base are on the order of M. In this case, there is so little acid and base, that even if the Kn value were large, the actual extent of reaction could still be small.

It is not that common that we would encounter such solutions in a laboratory setting where we usually use much higher concentrations. But this could occur in environmental samples for some species. Assuming solutions with appreciable concentrations of acid and base, would we ever have a small value of Kn?

Recollecting back, we talked about weak acids as having Ka values on the order of, from strongest to weakest, , , , and Similarly weak bases had Kb values from strongest to weakest on the same scale to For example, mixing an extremely weak acid with a Ka of with an extremely weak base with a Kb of will give a Kn of , a small number.

This neutralization reaction would not proceed much at all. If the acid had a Ka value of and the base a Kb value of , the value of Kn would be 1, an intermediate value. This neutralization reaction would proceed to some extent. If we considered a neutralization reaction in which either the acid or base was strong a strong acid or base might have a Ka or Kb value on the order of or higher , you would need the other species to have a K value of or lower to get a small value of Kn.

Since this is an unreasonably low value for the weak acid or base, any acid-base reaction that involves either a strong acid or a strong base will go to completion. The first step is to calculate the initial concentrations of Hcba and EA, remembering that mixing the two solutions dilutes each of the species.

If yes it will partially dissociate in water. Now you need to pay attention to the equilibrium reactions. First write out the balanced equilibrium reaction. Exactly how to solve this depends upon what other species are present. More on that coming up. Is it a salt? What will it do in solution. FIRST, the salt will dissociate into ions. Any other salt will do the same thing. You should be able to identify a salt by looking at he periodic table and thinking about trends in electronegativity.

First let the salt dissociate. Almost everyone in the class missed one or more problem because they failed to recognize a salt, and then see what it will do in solution. After you write the salt out as ions, look at the ions to see if you recognize any of them as a weak acid or a weak base. Or the conjugate acid or base. Anything with a Ka or Kb. After you recognize it as an acid or base, write the appropriate reaction.

This step can be a bit tricky. For the example here F1- is the conjugate base of a weak acid. The reaction that you write depends upon what species are present. The idea here is to think about what the strongest acid is. If only the salt is present, the strongest acid is water. This weakens the bond enough to make a water molecule surrounding each metal ion a proton donor.

The conjugate base of a weak acid reacts with water to form a basic solution. The fluoride ion, F1-, is the conjugate base of a weak acid.

Since the conjugate base of a weak acid is a stronger base than water, it will react with water to form a basic solution. After you recognize it's a base, write the appropriate reaction. This step can be a bit tricky since reaction that you write depends upon what species are present.

The idea here is to think about what the strongest acid is. If only the salt is present, the strongest acid is water. Since HF is a weak acid, F1- is also a weak base. That means you need to use an equilibrium expression to solve for the concentration of each species. We will deal with the equilibrium expressions below. Write out a list of all species present. After you identify all the species present and write out appropriate reactions, write out a list of all the species present, ignoring equilibrium reactionsFirst concentrate on the species present.

A strong acid will react with a strong base. The reaction will go to completion. Determine which is the limiting reagent. Find the final concentration of the excess reagent. The final concentration of the excess reagent will determine the pH. A strong acid will react with its conjugate base. If conjugate base is limiting, there is excess strong acid.

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Since it is the ions that do the chemistry, write them as ions. Or the conjugate acid or base. First identify each species and decide what it will do in water Is it a strong acid?
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If you only have a weak acid. Exactly how to solve this depends upon what other species are present. If we considered a neutralization reaction in which either the acid or base was strong a strong acid or base might have a Ka or Kb value on the order of or higher , you would need the other species to have a K value of or lower to get a small value of Kn. What we need to know is the extent of the neutralization reaction, in other words the value of K for this reaction. The idea here is to think about what the strongest acid is. If the weak acid is limiting, there is excess strong base.
Solving acid base problems
We can therefore use the appropriate Henderson-Hasselbalch equation for chlorobenzoic acid to solve for the pH of this solution. That means you need to use an equilibrium expression to solve for the concentration of each species. Determine the concentration of the conjugate base. Is it a weak acid? If we substitute these into the Henderson-Hasselbalch equation of ethylammonium we ought to get the same pH as above.

If the weak acid is limiting, there is excess strong base. If you only have a weak acid. Write out a list of all species present. As tempting as it might be to say hydronium and hydroxide will react to produce the water molecules thereby just cancelling these out and ignoring them , they are real species in the reaction that need to be accounted for in the final form of Kn. There are no tables of Kn values so what we need to do is see if there is a way to come up with the Kn expression by adding up a series of reactions that we do have K values for. The reaction that you write depends upon what species are present.
Solving acid base problems
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If only the salt is present, the strongest acid is water. Recollecting back, we talked about weak acids as having Ka values on the order of, from strongest to weakest, , , , and A strong acid will react with a strong base. If yes, it will completely dissociate in water. What you need to do is figure out the hydroxide in concentration, [OH1-]. Chlorobenzoic acid Hcba is a weak acid with a pKa value of 3.
Solving acid base problems
Identify Each Species. This determines the pH. What we need to know is the extent of the neutralization reaction, in other words the value of K for this reaction. If X is similar to [HA], solve with the quadratic equation.
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Reviews

Tojakazahn

Now you need to pay attention to the equilibrium reactions. Determine the concentration of the weak acid and the conjugate base Ignoring any equilibrium effects at first Look up or determine Ka.

Goltilmaran

Determine which is the limiting reagent. As tempting as it might be to say hydronium and hydroxide will react to produce the water molecules thereby just cancelling these out and ignoring them , they are real species in the reaction that need to be accounted for in the final form of Kn.

Kagagore

Can we always expect Kn to be large such that neutralization reactions always go to completion? The reaction that you write depends upon what species are present. Find the final concentration of the excess reagent. Exactly how to solve this depends upon what other species are present.

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