MCAT Physical : Acid-Base Chemistry
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Example Question #3 : Buffers
Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.
All of the bases proceed in a similar fashion.
The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.
A scientist is making a buffer to maintain the pH of a solution at about 6. Which of the following describes the best composition of the solution?
About equal amounts of acid and conjugate base, where the acid has a pKa of 5.9.
About equal amounts of acid and conjugate base, where the acid has a pKa of 6.9.
A larger amount of acid , with some conjugate base, where the acid has a pKa of 5.9.
A larger amount of acid , with some conjugate base, where the acid has a pKa of 6.9.
A larger amount of acid , with some conjugate base, where the acid has a pKa of 4.9.
About equal amounts of acid and conjugate base, where the acid has a pKa of 5.9.
A buffer is best made of equal and copious amounts of an acid and its conjugate base. The acid must have a pKa as close as possible to the pH desired. For this question, the desired pH is 6. The best option will have a pKa of about 5.9.
Adding larger amounts of the acid than the base will skew the equilibrium in favor of the acid, increasing the overall proton concentration, and limiting the buffer ability to absorb more proton addition without changing pH significantly.
Example Question #2 : Buffers
Which of the following would be a good buffer solution?
A good buffer solution consists of a weak acid with its conjugate base, or a weak base and its conjugate acid. A strong acid or base will never be a good buffer. We will go through each answer choice in detail.
Example Question #1 : Buffers
Which of the following pairs cannot constitute a buffer system?
A buffer system is composed of one of the following scenarios:
1. A weak base with its conjugate acid.
2. A weak acid with its conjugate base.
The only option that does not qualify is the nitrate, nitrite pair. A difference of one oxygen does not make a buffer system.
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