Learning Objective Define buffer and describe how it reacts with an acid or a base. Assume that all are aqueous solutions.
The combination of these two solutes would make a buffer solution. Hydrochloric acid HCl is a strong acid, not a weak acid, so the combination of these two solutes would not make a buffer solution. Because it is not on our list of strong bases, we can assume that it is a weak base.
The compound CH 3 NH 3 Cl is a salt made from that weak base, so the combination of these two solutes would make a buffer solution. The combination of these two solutes would not make a buffer solution. Answer b No. Need a weak acid or base and a salt of its conjugate base or acid.
Answer c Yes. Answer d No. As buffer works to resist changes in its pH, the concentration of one component decreases, while the concentration of the other increases.
To understand how buffers work, we must first discuss the concept of conjugates in acid-base chemistry? What is a conjugate? From this, you will notice that a conjugate pair always consists of an acid and a base. And when a proton is added to a base, the base turns into its conjugate acid. Thus, the acid and its conjugate base are related by the loss and gain of a proton. If we apply the concept of conjugate to buffer solutions, then we can say that a buffer solution usually consists of a weak acid and its conjugate base or a weak base and its conjugate acid.
However, not all acids and their conjugate bases can form buffer solutions. For instance, on paper, the conjugate base of hydrochloric acid HCl is the chloride ion Cl. 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:. Buffer Solutions Preparing a Buffer Solution with a Specific pH A buffer is a solution of weak acid and conjugate base or weak base and conjugate acid used to resist pH change with added solute.
Learning Objectives Describe the properties of a buffer solution. Key Takeaways Key Points Buffer solutions are resistant to pH change because of the presence of an equilibrium between the acid HA and its conjugate base A-. When some strong acid is added to a buffer, the equilibrium is shifted to the left, and the hydrogen ion concentration increases by less than expected for the amount of strong acid added.
Buffer solutions are necessary in biology for keeping the correct pH for proteins to work. Buffers can be prepared in multiple ways by creating a solution of an acid and its conjugate base. Equilibrium 1. Using the Henderson-Hasselbalch equation gives:. What is the effect of adding of 0. The strong acid, HCl would dissociate completely in water:. The new concentration would be:.
The equilibrium would then be re-established:. There is a small change in pH, from 4. This same amount of HCl added to water shows a much bigger effect. For example, we could make a solution that is 0. We can use the Henderson-Hasselbalch equation to calculate the pH of this buffering solution. Since we are calculating the pH, we have to treat this as an acid dissociation reaction.
These two examples should point out an important consideration in making buffers. The pH of a buffer is close to the p K a of the acid used in the buffer.
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