Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach you the buffers of Acids and Bases. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Buffers are aqueous solutions that minimize the pH change when acids or bases are added to that solution. When you take a buffer solution and add some acid or base, the pH will only change by a very small amount, resisting pH change unless a great quantity of acid or base is added. We will cover the characteristics of a good buffer and the calculations involved in understanding a buffer system.
Creating a Buffer
There are two different ways to create a buffer. A buffer system can either be produced by combining a weak acid and its conjugate base in solution, or by combining a weak base and its conjugate acid in solution. While a buffer system can be created by combining the conjugate pairs of weak acids and bases, it is not possible to form a buffer system using strong acids and bases, which ionize fully in solution.
Recall that the point of a buffer is to minimize the changes in pH to a solution when acid or base is added to it. There are several characteristics that make something a good buffer solution. First is a high concentration of the conjugate acid-base pair. The conjugate acid and base are responsible for reacting with excess acid/base added to solution, minimizing the amount of hydronium and hydroxide formed. A buffer with high concentration of conjugate acid and base can be said to have excellent buffer capacity. Secondly, we want to select an acid for our buffer that has a pKa close to our desired pH. This idea is elaborated on in another video, but for now we can note that a conjugate acid-base pair is most effectively able to buffer when the pH of the system is near the pKa of the acid.
Calculating the pH of a buffer system
Previously we calculated the pH of strongly acidic solutions and the pH of weakly acidic solutions. Calculating the pH of a buffer system requires a new method of calculation, one involving an important equation called the Henderson-Hasselbalch equation. In this case, [A-] is the concentration of conjugate base and [HA] is the concentration of weak acid in solution.
Example
What is the pH of a 1L aqueous solution made from adding three moles of acetic acid (pKa 4.8) to three moles of sodium acetate? The first step in solving this question is deciphering the type of calculation we will perform. We can note that acetic acid is not one of the strong acids we have memorized, nor is this a solution composed entirely of weak acid or weak base. Instead, it is a mixture of a conjugate acid-base pair, which is an example of a buffer solution. Once it has been determined that this is a buffer solution, the next step is recognizing which compound is the acid and which compound is the conjugate base. In this case, acetic acid is the acid and acetate is the base. Third, we should determine what our values for pKa and concentration are. The pKa is provided to us, but in order to find the concentration of acid and base we will need to divide the number of moles of each by the volume of the solution. For both acetic acid and acetate, 3 moles in a 1-liter solution yields a concentration of 3 M. The next step is to plug all of the values of concentration and pKa into the Henderson-Hasselbalch equation and solve for pH.
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