# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jeffmoonchop on January 16, 2017, 10:14:25 AM

Title: Solubility reporting
Post by: jeffmoonchop on January 16, 2017, 10:14:25 AM
I'm doing some solubility measurements of highly soluble organic acids by stirring in excess and taking 1ml saturated solution and allowing it to dry.

Would the solubility value in g/ml be the dry mass left over? Because what I think it should be is this:

If 1ml citric acid saturated in water was taken and it weighs 1.2435g and the dry mass is 0.811g then the volume of water that actually evaporated is the (dry mass - mass of solution)/density of solvent which is 0.9999 at 20C. So the volume of water actually evaporated is 0.4329ml. So the actual mass of citric acid that can be dissolved in 1ml water at 20C is actually (1/vol of evaporated solvent)*dry mass which is 1.869g/ml.

would this be the correct solubility or am i overthinking it?

thanks
Title: Re: Solubility reporting
Post by: AWK on January 16, 2017, 10:33:39 AM
Solubilities in chemistry usually are given as mass of solute in 100 g of solvent. In chemical technology solubility is given as percentage of solute in saturated solution. In physical chemistry (especially when solubility is low) molar, molal or mole fraction concentrations  are use to this purpose.
Title: Re: Solubility reporting
Post by: jeffmoonchop on January 16, 2017, 10:39:22 AM
yes but as a chemist would the method of obtaining the solubility value in g/ml be correct? I can convert to g/100g later.
Title: Re: Solubility reporting
Post by: Dan on January 16, 2017, 11:25:24 AM
would this be the correct solubility or am i overthinking it?

You need to be clear about how you want to express your measurement because it's not clear.

If you say a compound has a solubility of X g/mL, this means that in 1 mL of solution, there is X g of the compound.

A solubility of X g/mL does not mean the solubility is X g in 1 mL of solvent (because the volume of X g of compound and 1 mL solvent - i.e. volume of solution - is not 1 mL).

The same is true for % concentrations. 36% HCl is not 36 g HCl in 100 g of water, it's 36 g HCl in 64 g of water.

These measures express solubility of a compound relative to the whole solution, i.e. solute vs solution = solute vs (solute+solvent).

What you seem to be calculating is the ratio of solute vs solvent - this type of relationship is used in molality (moles solute vs mass solvent) and mass ratio, but I can't think of other examples.
Title: Re: Solubility reporting
Post by: jeffmoonchop on January 17, 2017, 05:04:29 AM
Yes thanks that makes sense. So the solubility would be the dry mass in g/ml without the need of further calculation. Do you have a reliable source that states thats its per ml of solution not solvent like the first reply stated?

cheers
Title: Re: Solubility reporting
Post by: AWK on January 17, 2017, 05:17:42 AM
Do you have a reliable source that states thats its per ml of solution not solvent like the first reply stated?

cheers
No source is needed. This is just recalculation of molar concentration [mol/L] multiplyed by molar mass of solute. The rest is simply scaling from g/L to g/ml.
Title: Re: Solubility reporting
Post by: Dan on January 17, 2017, 05:57:10 AM
Yes thanks that makes sense. So the solubility would be the dry mass in g/ml without the need of further calculation. Do you have a reliable source that states thats its per ml of solution not solvent like the first reply stated?

https://en.wikipedia.org/wiki/Mass_concentration_(chemistry)