All of the following acid-base pairs are examples of buffers EXCEPT: (Question from "Cracking the MCAT, 2010-2011 edition" by The Princeton Review)
HCl/NaCl
CH3CO2H/NaCH3CO2
HF/NaF
H2CO3/NaHCO3
I chose ch3co2h/nach3co2
What you picked is a very good buffer.
Do you know what a buffer is.
Acid buffers are WEAK acids and their salts.
Base buffers are WEAK bases and their salts.
You chose CH3COOH which is acetic acid and that is a weak acid. NaCH3CO2 is the salt of acetic acid so that makes a good buffer. What do you see in that list that is NOT a weak acid or weak base?
ok so i think its hcl/nacl
right. The definition will always tell you.
To determine which of the options is not a buffer, we need to first understand what a buffer is. A buffer is a solution that resists changes in pH when acids or bases are added to it. It is composed of a weak acid and its conjugate base (or a weak base and its conjugate acid).
Let's go through each option and identify whether it is a buffer or not:
1. HCl/NaCl: This is not a buffer because it consists of a strong acid (HCl) and its salt (NaCl). A strong acid fully ionizes in water and does not have a conjugate base. Therefore, it cannot resist changes in pH effectively.
2. CH3CO2H/NaCH3CO2: This option is a buffer. Acetic acid (CH3CO2H) is a weak acid, and its conjugate base (CH3CO2-) is derived from the full dissociation of the acid. Together, they form an effective buffer system.
3. HF/NaF: This option is a buffer. Hydrofluoric acid (HF) is a weak acid, and its conjugate base (F-) is derived when the acid partially dissociates. Thus, this combination can act as a buffer solution.
4. H2CO3/NaHCO3: This option is a buffer. Carbonic acid (H2CO3) is a weak acid, and its conjugate base (HCO3-) is formed by the partial dissociation of the acid. Both components make it a buffer system.
Based on the analysis, the option that is NOT a buffer is HCl/NaCl. It consists of a strong acid and its salt, which does not form an effective buffer system.
Therefore, your choice of CH3CO2H/NaCH3CO2 as the buffer system is correct.