Question: I wrote:

> 
> Hydrochloric ions can influence the mutual solubility of IL and water **and hence** affect the extraction mechanism.
> 
> 
> 

For several conclusions, I may use this construction. I would like to know how idiomatic is this structure, and what are other alternatives to use for such short conclusions in a sentence.

Answer: 
If you want to talk about the status of the toilet and you want to use word “use”, then the expression I would use is “in use”, meaning that the toilet is being used. Then the opposite would be “not in use”.

I’m guessing that you’re looking for one word though. We often say that the restroom (and by extension, the toilet) is *occupied*. In fact, [when we’re in the restroom and someone knocks, a common response is “occupied!”](https://ell.stackexchange.com/q/123195/36187) The opposite would be *unoccupied*.
The words on the toilet door "dial" are:

1. **Vacant** (i.e. available, free, unoccupied)
2. **Engaged** (i.e. unavailable, in use, occupied)