# Discrepancy between the apparent volume of the solution and the volume of the solute arising from the definition of solubility

I have solubility $$s$$ given in $$\pu{g/mL}.$$ I understand this as mass of solute $$m$$ divided by volume of solvent $$V.$$

But how about the volume of the resulting solution? I read online I should pour the solute into a measuring cylinder/cup first, and then add solvent until the volume fits the calculated value $$(V = m/s),$$ but this seems to be at odds with the definition of solubility.

What is the correct way to understand solubility? I can't know the volume of a solution based on solubility.

• In a typical scenario, I would start by adding some solvent to a volumetric flask, maybe 25 to 50% full. Then quantitatively add the weighed mass of the solute. Rinse any solute down into the flask, swirl to dissolve the solute, and then add solvent to the mark on the volumetric flask. A stirring bar, if used, has a small volume that has to be compensated and volumetric flasks are calibrated for a specific temperature. That is the nutshell version. – Ed V Mar 29 at 20:01
• I'll add that in general volumes are not additive when using mass/volume units. // There is a measure % v/v where a solution could be made by mixing volumes. Say 50% water and 50% ethanol by volume. However if you mix 50 mL of water with 50 mL of ethanol you won't get exactly 100 mL of solution. – MaxW Mar 29 at 20:11
• I am just wondering whether what i am doing is right. Calculating volume of solvent from solubility, then adding the solute. What's bothering me is, that of course, the volume of the solution is not the same as that of the solvent. For example, i mixed a kilo of solute into 650 mL of solvent (water) and the volume of the solution jumped to about 1000 mL. I didn't expect this, though I should have and I am wondering whether I am interpreting solubility correctly. How am I to know what volume of solution I will obtain? This is important so i can select the right container for the solution. – user1095108 Mar 29 at 21:45
• @user1095108 - See EdV's comment. You need to put the kg of solute in a 1 L volumetric flask, then add enough water to fill the flask about 2/3 full. Dissolve the solute, then add enough water to get to the 1 L mark on the flask. – MaxW Mar 29 at 23:50
• Shouldn't I add exactly as much volume of water, as the solubility equation says I should (650 mL) and then end up with whatever volume I end up? – user1095108 Mar 30 at 5:10

I have solubility s given in g/mL. I understand this as mass of solute m divided by volume of solvent V.

Your confusion is arising from mixing the concept of solubility and concentration. Unfortunately, both are expressed the same way. You have to see the context. Most authors (like good scientists) try to write their experiment explicitly so that others can understand and repeat their experiment.

See the examples in this link: Solubility Concepts

Let us say, we find from the literature that the solubility of NaCl in water is 360 g / 1L. It means that if we take 1 L of pure water, we should be able to dissolve 360 g of salt in it. Note by adding 360 g of NaCl, the volume of water may not be exactly what 1 liter anymore. I cannot calculate concentration out of this information.

Now some authors may refer to concentration when they talk about solubility. For very dilute solutions it does not make a difference. So it is always good to see the context.

I read online I should pour the solute into a measuring cylinder/cup first, and then add solvent until the volume fits the calculated value (V=m/s), but this seems to be at odds with the definition of solubility.

Don't trust the webpages made by amateurs or pages from non-reliable sources. With time you will develop the sense which sources are more reliable. There is a saying that paper never refuses ink. It means one can write whatever on paper, and the paper will never say no, even if it is wrong. The same is true for the web. If we search the web for corona virus, you will find a lot of non-sense. Only reliable sources are to be trusted.