Is there any vocabulary difference among countries in Latin America?
Yes. the exact same word can mean different things depending on where you go. For example 'cola' in Ecuador means coke as well as a tail. But if you tell 'cola' to Mexicans, they will never guess that you're talking about coke because for them is 'gaseosa'.
Why do we find NaCl (Sodium Chloride) in nature rather than sodium or chlorine separately?
Because Chlorine lacks one electron in its electronic configuration to achieve the electronic configuration that Argon has. Always a noble gas has the most stable energetic state. And Sodium has one extra electron if we compare it with the electronic configuration of Neon (another noble gas). To be stable, both (chlorine and sodium) need each other. Sodium chloride is more stable than sodium and chlorine separately.
Can you tell the difference between you (singular) and you (plural)?
Well, you have to pay attention to the context. For example the question 'What are you doing tonight?' can be interpreted in both ways. But if I know that there was only one person previously, I would know that you are referring to the singular form. On the other hand, if I know that I was asking the question to a couple, I know that you are referring to the plural form