Final answer:
When CH₃CHO acts as a base and reacts with HCl, it forms its conjugate acid CH₃CHOH+, where oxygen in CH₃CHO accepts a proton from HCl.
Step-by-step explanation:
The molecule CH₃CHO acts as a base, and when it reacts with HCl, a conjugate acid is formed. The oxygen of the CH₃CHO has a lone pair which can accept a proton (H+), so when it reacts with HCl, the oxygen will bond with the additional proton.
This results in the formation of CH₃CHOH+, which is the conjugate acid of the base CH₃CHO. In a conjugate acid-base pair, the acid donates a proton in the forward reaction, while the base accepts a proton in the reverse reaction. Therefore, the conjugate acid of CH₃CHO after reaction with HCl is CH₃CHOH+.