How can you notify when a functional group will act as an acid or a base? an act of

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How can you tell when a functional social will act as an acid or a base?

I know that the presence of H generally means it will act as an acid, but then there are groups like amino groups, where NH2 acts as a centre, becoming NH3. And does -OH (hydroxyl) act as a base or an acid?
I just really don't understand how you tell when a group will act as an acid or a bottom when attached to some other compound. Please explain if you understand!

If a group can accept hydrogens it will act like an base, if it donated hydrogens it will act equal an acid. In the case amino groups they can accept H+ to become R-NH3+ if they then give it away in an reaction they are then acids. With -OH they can accept H+ to form H2O and there for are bases. H2O + H+ on the other aid is not a base although it does form H3O+ which is an acid.

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