The Portuguese don't seem to use the definite article with "França", they say "Embaixador de França", for example - just another Portugal/Brazil difference I suppose.
According to our Portuguese teacher, for Inglaterra, Espanha, França & Italia the article is optional.
That's interesting. I don't think that's true in Brazilian Portuguese, though, based on Paulenrique's answer here and this list: http://www.portuguesehub.com/grammar/definite-article-with-country-names/.
Both 'rei de França' and 'rei da França' are correct. But the first one is more common in Portugal and the 2nd in Brazil.
One of the sentences in this said, "Amanhã eu explico porque" I got it wrong because I wrote "por que" instead of "porque". I have been googling it and everyone says that it is the same as Spanish where "porque" means "because" and "por que" means why. Mistake on the site or on google? Certain rule that I don't know or something? I'm confused.