Certain about or of ?
Is there any difference between certain about/of ?

Here are some examples that I've found on the internet.Number 1 and 2 look so similar but I'm not sure about the third one.

1-) Now,are you certain about that?
2-)They were watching him.He was certain of it.
3-)It's wise to apply early to be certain of obtaining a place.

Could you make it clear for me ?

Question

Certain about or of ?
Is there any difference between certain about/of ?

Here are some examples that I've found on the internet.Number 1 and 2 look so similar but I'm not sure about the third one.

1-) Now,are you certain about that?
2-)They were watching him.He was certain of it.
3-)It's wise to apply early to be certain of obtaining a place.

Could you make it clear for me ?

Answer

There is no difference, however "certain of" is used most commonly, so if you're ever unsure, use that.

Answer

Keep in mind that prepositions create a relationship between their object and another word in the sentence, usually the main verb or the noun before the preposition. About and of are not always interchangeable, it depends on the relationship they are creating. 
I'm concerned about Max.
*I'm concerned of Max. (*incorrect)
I'm certain of Max. (A little unclear without more context but acceptable grammar and meaning)
I'm certain about Max (A little unclear without more context but acceptable grammar and meaning)

So sometimes different prepositions can create the same relationship (meaning) but not always.

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