I was doing this and thought that too. It does sound more natural but "Ii want tomato soup." in portuguese would be "Eu quero sopa de tomate." It's a slight difference. It might show I have no points in any language but I speak fluent portuguese I lived there for 6 years.
So is this sentence more applicable to when you are ordering at a restaurant? In the US, "I want a tomato soup" would likely mean the same thing as "I want tomato soup" in just about any context, but the former would sound a bit odd in just about any scenario except ordering. In Portuguese, is the latter simply the more "generic" version?
I'm brazilian - gf of this canadian guy - and I came to help him with a couple things in portuguese. I know it might look wrong but it how it is. When we are learning english and we cant put an A in front of some words, it seems weird too. But theres differences on a way how languages communicate. Thats how it is (:
So, if I am understanding you correctly, "Eu queto uma sopa de tomate" is the common and correct way to say you want a bowl of tomato soup, yes?
If so, then the translation should be either "I want tomato soup" or "I want a bowl of tomato soup" because those, while not word-for-word literal, are the common and correct ways to say the same thing in English.