Something about the idea of Molarity is confusing me

Okay, so molarity is defined as **moles of solute per liter of solution**. It is used to describe the concentration. So in the lab, when I see a bottle labeled something like $\mathrm{3.5~M}~\ce{H2SO4}$, is it saying that the **only** chemical in the bottle is this $\ce{H2SO4}$? 


Because the way I think of it is "there are 3.5 moles of $\ce{H2SO4}$ per liter of solution". The word "solution" is kind of throwing me off because it makes me feel like they are saying that there is more than just $\ce{H2SO4}$ in the bottle. Correct me if I'm wrong, but don't you need at least 2 solutes to make a solution? 


I don't know why I'm having such a hard time grasping this. But any help would be awesome! 





> 
> Correct me if I'm wrong, but don't you need at least 2 solutes to make a solution?
> 
> 
> 


You are, unfortunately, incorrect. In wet chemistry, you need just a *solvent*$^\dagger$ and at least one *solute* to make a solution. While this does mean that you need at least two *constituents* to make up a solution, you do not need at least two *solutes*.



> 
> ... is it saying that the ONLY chemical in the bottle is this $\ce{H2SO4}$?
> 
> 
> 


It depends on your definition of a *chemical*:


* If you consider water to be a chemical, then ***no***: there are two chemicals in the bottle$^\ddagger$, $\ce{H2O}$ and $\ce{H2SO4}$.
* If you are not considering water to be a chemical, then ***yes***: the only chemical in the bottle$^\ddagger$ is $\ce{H2SO4}$.


$^\dagger$ If not specified, the solvent is almost always water.


$^\ddagger$ Except for small amounts of inevitable impurities.


 7

Brian has the correct answer.


I'd like to add, however, that in the most general sense, we don't have to take a molar to mean a mole of **solute** per liter of **solution**. I've actually seen it used to mean "a mole of **stuff** per liter of **space**" for particular values of stuff.


Since the above sentence isn't exactly self-explanatory, let's take a look at a few examples.


### Example 1: Water


At 4 °C, water has a density of 1000 g/L. Water has a molar mass of 18.02 g/mol. This means that in a liter of water at 4 °C, there are:


$$
1~\mathrm{L} \cdot 1000 ~\frac{\mathrm{g}}{\mathrm{L}} \cdot \frac{\mathrm{mol}}{18.02 \mathrm{g}} = 55.5 ~\mathrm{mol}
$$


Since there are 55.5 moles of water per liter of water, we can actually say that, at 4 °C, water is 55.5 molar. (See [Example 3 on Wikipedia](https://en.wikipedia.org/wiki/Molar_concentration#Examples)).


Note how this works: there is a certain amount of **stuff** (in this case, water) per amount of **space** (one liter, or one cubic decimeter). It doesn't matter that we don't have a solution--since there's stuff, and it takes up space, we can calculate a molarity for it.


### Example 2: Ideal gas


An ideal gas takes up 22.4 L per mole, at STP. Taking the inverse of this, we find that it is 0.0446 moles/L. Therefore, the concentration of an idea gas at STP is 0.0446 molar.


Again, there's a certain amount of stuff (one mole of ideal gas) and a certain amount of space (22.4 liters), which allows us to calculate a molarity. The fact that the substance is pure doesn't stop us from writing a concentration for it.


### A note of caution


This will **NOT** work for a measure of concentration like molality, because molality is defined by the amount of **solvent** used. It also won't usually work for things like normality, since you (usually) need to be in solution to generate ions.


 6

I think this is the best way to clear your doubt. Have you ever prepared tea? What are the main ingredients of tea?


$$\ce{water~(or~milk) + tea~leaves -> tea}$$


It's the addition of tea leaves which changes water to tea. Depending on how strong you want a tea you can add one teaspoon, two teaspoons or n number of teaspoons of tea leaves.


Similarly, in $\mathrm{3.5~M}~\ce{H2SO4}$ solution it contains:


$$\ce{H2SO4~(solute) + H2O~(solvent) -> H2SO4~solution}$$


To my knowledge there is no 100 percent pure $\ce{H2SO4}$ available commercially. The commercially available $\ce{H2SO4}$ is 96 percent $\ce{H2SO4}$ with a concentration approximately of 18 molar. See [this link](http://crescentok.com/staff/jaskew/isr/safety/cabconc.htm).


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