This もの is one of the Japanese particles. We call this kind of particle 終助詞（しゅうじょし）. Question word かis also one of this. もの is the one which shows reason or exclamation. About your sentenses, the first one shows the meaning of reason, and the second one shows, I'd say, both meanings.
上着を脱いでもいいですか。暑いものですから。 If you use both もの and から, it sounds you emphasize on reason a lot.
パーティには行かなかったよ。知らなかったもの。
I didn't go to the party, because I din't know.（no one told me about party. i'm kind of angry)
もん is, as Chi さん above mentioned , a casual way to say もの. But you have to be careful to use もん, because this もん sometimes sounds girlish or こどもっぽい.