The difference lies in the length of how long a person is unconscious.
Pass out: short period e.g. faints
Pass way: long period e.g. dies
Take for example, a horror movie.
Someone is walking in a dark house. Suddenly, a ghoul jumps out in front of them. They are shocked, and they faint.
= (they have) passed out
Someone is walking in a dark house. Suddenly, a ghoul stabs them. Their blood spills, and they die.
= (they have) passed away
I do realise you have posted this 2 years ago, but an answerable question shall be answered, or else the question remains single and lonely. #poeticaf #excuseme