Difference between もの／もん and ものだから

Hi Mikel it's always nice to meet another Australian at Italki. These 2 websites may be useful for you:
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www.englishjapaneseonlinedictionary.com/
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Japanese to English Translation
Translate Japanese to English online and download now our free translator to use ... into English by Japanese to English translation dictionaries. ...
translation.babylon.com/Japanese/to-English/

I'm going to give my expression by the senteses you wrote.
I think you understand them as "because" in English.
The first one sounds formal and the second one sounds casual.
I think もんだから is for spoken Japanese or casual way.

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.