First, let’s deal with the pronunciation. The `a’ sounds like the `a’ in `act’, cat’, and `hat’; the following `o’ is like the `a’ in `china’. The final `o’ is pronounced like the `oa’ in `coat’, `boat’, and `goat’; the `s’ is silent. The stress is on the final syllable.
The expression `apropos of’ is usually used to introduce a new subject, but one which is connected to what you were talking about earlier.
Example: I received a call from Priyanka last night — apropos of which, did you send her the paintings?
When you say that something is `apropos’, you mean that it is suitable for the situation.
Example: The Vice Chancellor’s remarks were very apropos.