Again, since most of the OH - is neutralized, little pH change will occur. Buffers function best when the pK a of the conjugate weak acid used is close to the desired working range of the buffer. This turns out to be the case when the concentrations of the conjugate acid and conjugate base are approximately equal within about a factor of For example, we know the K a for hydroflouric acid is 6.
For the weak base ammonia NH 3 , the value of K b is 1. It's always the pK a of the conjugate acid that determines the approximate pH for a buffer system, though this is dependent on the pK b of the conjugate base, obviously. When the desired pH of a buffer solution is near the pK a of the conjugate acid being used i. In this example we will continue to use the hydrofluoric acid buffer.
We will discuss the process for preparing a buffer of HF at a pH of 3. This is simply the ratio of the concentrations of conjugate base and conjugate acid we will need in our solution. How much Sodium Fluoride would we need to add in order to create a buffer at said pH 3. From a table of molar masses, such as a periodic table, we can calculate the molar mass of NaF to be equal to Using this information, we can calculate the amount of F - we need to add. We could use ICE tables to calculate the concentration of F - from HF dissociation, but, since K a is so small, we can approximate that virtually all of the HF will remain undissociated, so the amount of F - in the solution from HF dissociation will be negligible.
Thus, the [HF] is about 1 M and the [F - ] is close to 0. This will be especially true once we have added more F - , the addition of which will even further suppress the dissociation of HF.
Thus, [F - ] should be about 0. For mL of solution, then, we will want to add 0. If the concentrations of a solution of a weak acid and its conjugate base are reasonably high, then the solution is resistant to changes in hydrogen ion concentration.
These solutions are known as buffers. It is possible to calculate how the pH of the solution will change in response to the addition of an acid or a base to a buffer solution.
A solution is 0. Calculate the change in pH when 0. Compare this to the pH if the same amount of HCl is added to a liter of pure water. Therefore, the solution will contain both acetic acid and acetate ions.
In this example, ignoring the x in the [C 2 H 3 O 2 — ] and [HC 2 H 3 O 2 ] terms was justified because the value is small compared to 0. Then, we consider the equilibrium conentrations for the dissociation of acetic acid, as in Step In the presence of the acetic acid-acetate buffer system, the pH only drops from 4.
A formic acid buffer is prepared with 0. The K a for formic acid is 1. What is the pH of the solution? What is the pH if 0. What would be the pH of the sodium hydroxide solution without the buffer? What would the pH have been after adding sodium hydroxide if the buffer concentrations had been 0.
The pH went up from 3. Solving for the pH of the buffer solution if 0. This shows the dramatic effect of the formic acid-formate buffer in keeping the solution acidic in spite of the added base. It also shows the importance of using high buffer component concentrations so that the buffering capacity of the solution is not exceeded.
An alkaline buffer can be made from a mixture of the base and its conjugate acid, but the formulas for determining pH take a different form. These compounds are generally weaker bases than the hydroxide ion because they have less attraction for protons. For example, when ammonia competes with OH — for protons in an aqueous solution, it is only partially successful.
Reactions with weak bases result in a relatively low pH compared to strong bases. Bases range from a pH of greater than 7 7 is neutral like pure water to 14 though some bases are greater than An alkaline buffer can be made from a mixture of a base and its conjugate acid, similar to the way in which weak acids and their conjugate bases can be used to make a buffer.
The formula for pOH is:. Weak bases exist in chemical equilibrium much in the same way as weak acids do. A base dissociation constant K b indicates the strength of the base. For example, when ammonia is put in water, the following equilibrium is set up:.
Bases that have a large K b will ionize more completely, meaning they are stronger bases. As the bases get weaker, the K b values get smaller. Calculate the pH of a buffer solution consisting of 0. Privacy Policy. Skip to main content. Acid-Base Equilibria.
Search for:. You can change the pH of the buffer solution by changing the ratio of acid to salt, or by choosing a different acid and one of its salts. Alkaline buffer solutions have a pH greater than 7 and are made from a weak base and one of its salts. A very commonly used example of an alkaline buffer solution is a mixture of ammonia and ammonium chloride solution.
If these were mixed in equal molar proportions, the solution would have a pH of 9. Similarly when NaOH strong base is added to this buffer system, the ammonium ion donates a proton to the base to become ammonia and water thus neutralizing the base without any significant pH change.
The further addition of an acid or base to the buffer will change its pH quickly. Thus the breaking of the buffer is its capacity, or in other words, it is the amount of acid or base, a buffer can absorb before breaking its capacity.
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