Conjugate base contains one less proton as compared to it's parent acid. Deprotonation occurs from most acidic region.
Propionic acid deprotonates to produce it's strong conjugate base propionate.
In propionic acid, -OH group in -COOH functional moiety dissociates to produce . Because the O-H bond electrons remains highly polarized towards oxygen atom due to electron withdrawing inductive effect as well as resonating effect of -COOH moiety.