Select the correct answer.
Which statement correctly relates two substances from this reaction?
[tex]H_2 CO_3+H_2 O \rightleftharpoons H_3 O^{+}+HCO_3^{-}[/tex]
A. [tex]HCO _3{ }^{-}[/tex] is the conjugate acid of [tex]H _2 CO _3[/tex].
B. [tex]H _2 O[/tex] is the conjugate base of [tex]H _2 CO _3[/tex].
C. [tex]H _3 O ^{+}[/tex] is the conjugate acid of [tex]H _2 O[/tex].
D. [tex]H _2 CO _3[/tex] is the conjugate base of [tex]H _3 O ^{+}[/tex].
Answer (2)
Identify the acid and base in the reaction: H 2 C O 3 is the acid and H 2 O is the base.
Determine the conjugate base and acid: H C O 3 − is the conjugate base of H 2 C O 3 , and H 3 O + is the conjugate acid of H 2 O .
Check the options and select the correct one.
The correct answer is: C .
Explanation
Understanding Conjugate Acid-Base Pairs In this problem, we are given a chemical reaction and asked to identify the correct conjugate acid-base pairs. A conjugate acid-base pair consists of two substances that differ by a proton ( H + ). An acid is a substance that donates a proton, and a base is a substance that accepts a proton. When an acid donates a proton, it forms its conjugate base. When a base accepts a proton, it forms its conjugate acid.
Identifying Acid-Base Pairs The given reaction is:
H 2 C O 3 + H 2 O ⇌ H 3 O + + H C O 3 −
In the forward direction:
H 2 C O 3 donates a proton to become H C O 3 − . Thus, H 2 C O 3 is an acid and H C O 3 − is its conjugate base.
H 2 O accepts a proton to become H 3 O + . Thus, H 2 O is a base and H 3 O + is its conjugate acid.
Evaluating the Options Now, let's examine the given options:
A. H C O 3 − is the conjugate acid of H 2 C O 3 . This is incorrect. H C O 3 − is the conjugate base of H 2 C O 3 .
B. H 2 O is the conjugate base of H 2 C O 3 . This is incorrect. H 2 O is a base that becomes H 3 O + .
C. H 3 O + is the conjugate acid of H 2 O . This is correct. H 2 O accepts a proton to form H 3 O + .
D. H 2 C O 3 is the conjugate base of H 3 O + . This is incorrect. H 2 C O 3 is an acid.
Conclusion Therefore, the correct statement is that H 3 O + is the conjugate acid of H 2 O .
Examples
Understanding conjugate acid-base pairs is crucial in many chemical and biological processes. For example, in blood, the carbonic acid-bicarbonate buffer system ( H 2 C O 3 / H C O 3 − ) helps maintain a stable pH level. Carbonic acid ( H 2 C O 3 ) can donate a proton to form bicarbonate ( H C O 3 − ), which can then accept a proton to reform carbonic acid. This buffering action prevents drastic changes in blood pH, which is essential for the proper functioning of enzymes and other biological molecules.
In the reaction H 2 C O 3 + H 2 O ⇌ H 3 O + + H C O 3 − , H 2 C O 3 acts as an acid while H 2 O is a base. The correct relationship is that H 3 O + is the conjugate acid of H 2 O . Therefore, the correct answer is Option C.
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