# Concentration by composition?

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1. Nov 29, 2018

### somasimple

Hi All,

Is it possible to know the concentration of a solute when you have only its composition?
I.e : the solute contains 1 molecule of water for each k+

2. Nov 29, 2018

### Staff: Mentor

Unless my English fails me "solute" is the substance that is dissolved. Do you mean "solute" or "solution"?

In general the answer is "yes", as concentration is a way of expressing the mixture composition, so these things are interchangeable. But the devil is in the details. If there is one molecule of water per each molecule of ethanol we have solution that is 71.9% w/w in ethanol. In your case though the information is incomplete, solution must contain a conterion to be electrically neutral. Not knowing what the counterion is we can't find the concentration.

3. Nov 29, 2018

### somasimple

Thanks,
I meant solution of course (excuse my bad English)
it is just an hypothetical example of computation. I know that K+ solution doesn't exist fortunately (?).
Suppose it is KCl 1:1:1

Last edited: Nov 29, 2018
4. Nov 29, 2018

### Staff: Mentor

1 molecule of KCl (no such thing as KCl molecules, but it is not a problem here) per 1 molecule of water means 1 mole of KCl per 1 mole of water. That's well beyond KCl solubility, but technically calculating concentration (at least as w/w, or molality) is trivial.

5. Nov 29, 2018

### somasimple

For a chemist for sure but I'm not a chemist at all.
Perhaps a useful link or a bit of help?

6. Nov 29, 2018

### Staff: Mentor

Do you know what mole is, and what molar mass is? Do you know how mass percentage is defined?

(These thing are taught in chemistry class, but they are much more universal and quite common in other places as well).

7. Nov 29, 2018

### somasimple

The only thing I know is the mole = Avogadro number * atom or molecule mass.
BTW, I'm an old student of 61' so I left classes in 1980 ;-)

8. Nov 29, 2018

### Staff: Mentor

9. Nov 30, 2018

### somasimple

How do I convert a mass percentage to a mole/litre value?

10. Nov 30, 2018

### Staff: Mentor

You can't, unless you know the solution density (necessary to find the volume). In general predicting them from the first principles is next to impossible, but we have density tables for many substances.

11. Nov 30, 2018

### somasimple

Problem!
If I know that a KCL solution is, by example, 4 mmol, is it supposed to be 4 10-3 mole of KCl diluted in one litre of water or not?
With the same known concentration may I go to a relation/computation where there is n water molecules for each K or Cl ion?

12. Nov 30, 2018

### Staff: Mentor

4 mmol is ambiguous in this context, as it is not concentration - it is amount of the substance. But assuming it is a shorthand notation for a 4 mM (where M stands for mol/L) you are right, it means there are 4×10-3 moles of KCl per 1 L of solution.

You mean if it is possible to calculate n for the given molar concentration? Yes. Again, you will need to know the solution density for that. But for most practical applications mM solution is diluted enough so that its density is not far from that of a pure water. That means you can safely assume 1 L to weight 1000 g. Part of that is 4 mmoles of KCl (mass easy to calculate from molar mass KCl), rest is water (again easy to convert to number of moles this time using molar mass of water). Molar ratio is then identical to the molecular ratio.

13. Nov 30, 2018