How to differentiate between strong acid and weak acid!

[@andrewjustin256]

Hey there, Mr. Guru of Chemistry. I would like to inquire you how I may ask you questions. I'm enrolled in A Level Chemistry and I sometimes would like to ask specific questions. Would you be able to assist me on it? For instance, I would like to ask: I was supposed to predict the reaction of phosphorus (V) chloride (PCl5) and water and I predicted HCl and H3PO4 as shown: PCl5 + 4H2O -> 5HCl + H3PO4 Only I found out that it was not what I predicted; the correct reaction is: PCl5 + H2O -> 2HCl + POCl3 Why doesn't the first reaction occur instead of the second one?