Question

Identify conjugate acid and conjugate base for \( HCO_3^- \) ion respectively:

• A. Carbonic acid and carbonate ion
• B. Carbonate ion and carbonic acid
• C. Hydronium ion and hydroxide ion
• D. Water and carbon dioxide

Hint:

Remember: \[ \text{Conjugate Acid} = \text{Species} + H^+ \] \[ \text{Conjugate Base} = \text{Species} - H^+ \] Always carefully check the charge after adding or removing a proton.

Answer 1

The Correct Option is A

Concept: According to the Brønsted-Lowry acid-base theory:
• A conjugate acid is formed when a species accepts a proton (\(H^+\)).
• A conjugate base is formed when a species donates a proton (\(H^+\)). The ion \( HCO_3^- \) is called the bicarbonate ion and is an amphiprotic species, which means it can behave both as an acid and as a base depending upon the reaction conditions.

Step 1: Finding the conjugate acid of \( HCO_3^- \).
To obtain the conjugate acid, the given species must accept one proton (\(H^+\)). \[ HCO_3^- + H^+ \rightarrow H_2CO_3 \] Here, the bicarbonate ion gains one proton and forms carbonic acid. Therefore, the conjugate acid of \( HCO_3^- \) is: \[ H_2CO_3 \]

Step 2: Finding the conjugate base of \( HCO_3^- \).
To obtain the conjugate base, the given species must donate one proton (\(H^+\)). \[ HCO_3^- \rightarrow H^+ + CO_3^{2-} \] After losing one proton, bicarbonate ion forms carbonate ion. Therefore, the conjugate base of \( HCO_3^- \) is: \[ CO_3^{2-} \]

Step 3: Matching with the given options.
From the above calculations: \[ \text{Conjugate Acid} = H_2CO_3 \] \[ \text{Conjugate Base} = CO_3^{2-} \] Hence, the correct pair is: \[ H_2CO_3 \text{ and } CO_3^{2-} \]

Step 4: Evaluation of the given options.}
• \( CO_3^{2-} \) and \( H_2CO_3 \): Incorrect order.
• \( H_2CO_3 \) and \( CO_2 \): Incorrect because \( CO_2 \) is not the conjugate base.
• \( CO_2 \) and \( H_2CO_3 \): Incorrect pair.
• \( H_2CO_3 \) and \( CO_3^{2-} \): Correct pair.