Trivial name of a compound vs its chemical formula I had a question like below and I've had trouble with similar problems before and how to go about solving for the chemical formula.


  What is the chemical formula for lithium aluminum hydride?


The first thing that comes to mind, we have one lithium, one aluminum and one hydride. Then why is the chemical formula $\ce{LiAlH4}$? Where did the four $\ce{H}$ atoms come from? Earlier on in the course we had stuff like 


  What is the chemical formula for di-arsenic trioxide?


And that one is easy, I have 2 arsenic and 3 oxygen. Given the full name I can easily deduce the formula (or the other way around). But what do you do in situations where you don't get "all the information". Do I go about thinking up how the structure works and see that it somehow logically follows that it needs to be four $\ce{H}$? 

I know most basic compounds, e.g. sulphate is $\ce{SO4^2-}$, but I feel like I can't memorize some of the things. There has to be a more general way to do this. It feels like and I must have missed it. Essentially, the full systematic names are no problem, but the trivial names are giving me a rough time and I'm scared if they come up during my finals next week. Any tips regarding that?
No you are absolutely right here, but even sulphate or nitrate is wrong. If you take a closer look at what a sulphate essentially is, you will see the following: It is a separate unit, it has a central atom and ligands and the whole unit is negatively charged. If this was a metal we would say it's a complex and name it with some sort of -ate in the name. And this is exactly what you see here. So tetraoxosulfate in short is sulfate. Whit this shorter notation you are just not able to guess it unless it is so common that you should know about it. For lithium aluminium hydride it's the same thing, it should be called lithium tetrahydroaluminate. 

Now you may ask yourself how are you supposed to work with these problems? In all scientific cases just google it. That's what we do as well, if this is class or homework related, then your teacher will only give you certain examples. If he doesn't, talk to him about it.