Dear reader. In life, you have a lot of carbon. Life itself is organic carbon, after all. As humans, we respire, taking in oxygen and giving out carbon dioxide, in a glorious contribution to the carbon cycle.
Presenting to you, the cumulation of all these representations. Of carbons, oxygens, and of carbons double-bonded to oxygens.
The carbonyls.
Q1: both oxygen in carboxylate are replaced by O18.
Q3, it is not stable for a carbon to attach to halogen and hydroxyl group at the same time.