In the Brønsted–Lowry model, what is the conjugate base of $HCN$?

• A. $CN-$
• B. $H_{2}CN+$
• C. $NC-$
• D. $H{C}^N$

Answer: (A)

Verified step by step guidance

1. Recall that in the Brønsted–Lowry acid-base theory, an acid is a species that donates a proton (H⁺), and its conjugate base is what remains after the acid loses that proton.
2. Identify the acid given in the problem, which is HCN (hydrogen cyanide). This molecule can donate a proton (H⁺) from the hydrogen atom attached to the carbon-nitrogen group.
3. To find the conjugate base, remove one proton (H⁺) from HCN. This means subtracting an H⁺ ion from the molecule, leaving behind the species CN⁻.
4. Write the formula for the conjugate base after the proton is lost. Since the proton is removed, the conjugate base is the cyanide ion, represented as CN⁻.
5. Confirm that the conjugate base has one less hydrogen and carries a negative charge due to the loss of the positively charged proton, consistent with the Brønsted–Lowry model.