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For example the solution contains $\pu{0.5 mol}$ of a solute and the total volume is $\pu{0.5 L}$, the molarity would be $\pu{1M}$. But if I take a portion of that solution, let's say $\pu{0.1 L}$, would that molarity be still $\pu{1M}$? If it were that way, there would be $\pu{0.1 mol}$ of the solute in the portion.

Or, do the moles of the solute stay the same, $\pu{0.5 moles}$, and the molarity would change to $\pu{5M}$?

So my question is, does the amount of solute decrease when considering only a portion of the solution?

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    $\begingroup$ If the concentration of the whole soln is 1M, then a portion taken out remains 1M unless you do something to it. $\endgroup$
    – Waylander
    Nov 27 '18 at 18:06
  • $\begingroup$ So the solute would be 0,1 moles in 0,1L of the the dissolution, right? Because the concentration would be 1M? $\endgroup$ Nov 27 '18 at 18:08
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    $\begingroup$ Correct. If you concentrate it down to dry you would expect to get 0.1 moles of the solute $\endgroup$
    – Waylander
    Nov 27 '18 at 18:22
  • $\begingroup$ Or if I take out a portion of it to mix it with another dissolution, right? $\endgroup$ Nov 27 '18 at 18:24
  • $\begingroup$ Exactly so. If you take out 50ml to react with something else, that 50ml is at a concentration of 1M $\endgroup$
    – Waylander
    Nov 27 '18 at 18:25
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Molarity is simply a ratio between the number of mols of a solute within a liter of solvent. So, a 1M solution tells us that there is one mol of solute per liter of solvent. If I take half a liter, 0.5 L, I will also take along with that half-liter 0.5 mols of solute. However, the molarity hasn't changed, as it is still 1M.

If your question is: are there less moles in a solution of lesser volume, compared to a solution with greater volume, but with equal concentrations of the same solute? The answer is yes, the ratio will not change assuming we haven't added or removed any volume of the solvent. A solution of 1M and 5L has 5 mols. A solution of 1M and 2L has 2 mols, and so on.

If your question is: if I have the same amount of mols, but I decrease volume of solvent, does my concentration go down? The answer is no. Concentration would increase. If I concentrate a 1M solution that has 1 mol and 1 L by fixing the amount of mols and removing solvent, say to 0.5 L, my molarity would increase to 2 M, thereby increasing the concentration. We can get rid of solvent by boiling it off, which allows us to increase the concentrations of solutions without removing any mols of solute.

Though this question is old, this answer may help others. My advice is to come up with some arbitrary concentrations, mols, volumes, etc, and see what happens as each variable changes.

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Yes , the solute particles are homogeneously spread throughout solution so when we take a spoonful of solution , the number of solute particles in the spoon is less than those which were present in solution.

Molarity is an intensive property of a solution so it remains same no matter you calculate it by taking a solution filled beaker or by taking only a spoonful amount.

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No, solute will not decrease if volume will decrease.But molarity will increase for sure, cause molarity is the amount of solute in litre of solution.

Consider container A in which you took 1 litre of water and added two spatulas of NaCl and in another container B you took 0.5 litre of water and added two spatulas of NaCl. Now which container has greater amount of solute , the second one that is container B ,hence molarity of container B solution will be greater than container A.

Hope it will work to clear concept.

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    $\begingroup$ Yeah I get that, but I'm talking about only ONE glass of a dissolution. $\endgroup$ Nov 27 '18 at 18:41
  • $\begingroup$ Well, no. Unless you are doing a different experiment. If you have a homogeneous solution the molarity remains the same unless you do something to it. If you dissolve the same amount of solid in a smaller amount of liquid then you are right, but that wasn't the question. $\endgroup$
    – matt_black
    Nov 27 '18 at 21:18

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