Re: molarity

> So if, in a solution, we have 3 M Na+ and 1 M CO3- is the molarity of this
> solution:
>
> 2 M: 1 M Na2CO3 + 1 M Na+ that is left = 2M
> or
> 1 M: because Na2CO3 is the only thing I should considered

Neither, really. You seem to have an idea that "molarity" is a
property that can be applied to a solution as a whole. If you had a
complicated mixture of many solutes, like, say, seawater or blood, you
would rarely say "this is a 2.0 M solution", as if that encompassed
everything. There are times when it is useful to know the total
concentration of every blessed thing in solution. Most often, we speak
of the concentration of a specific component.

In your example, it is best to say it has 3 M Na+, 1 M carbonate (and 1
M of some undisclosed anion(s) to balance the extra sodium cations,
assumed charge -1 for simplicity). You may also describe it as 1 M
Na2CO3 and 1 M NaX (where X is whatever your undisclosed anion is). If
it were important to know the total of everything, you might say, this
solution has 5 M of total ions (3 M Na+, 1 M carbonate, 1 M X-).
Sometimes people care about ionic strength, which is just a slightly
different way of doing the accounting (sum up concentration times
charge squared for each solute).

In any event, it is never correct to say, without some context, "this
is a 2 M solution". You must always specify 2M of *what*. It may be a
specific solute or ion. If you want/need to report the total of
everything, say 2 M of total ions or whatever. You might have a
conversation like, "How strong is that HCl?" "It is 2 M." In this
case, it is clear from context that the solution is 2 M HCl (i.e. 2 M
H+ and 2 M Cl-). Most of the time, we don't care about the total of
all ions/solutes and are just interested in the concentrations of
specific components.

I hope this makes sense.

- Craig

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