Imran - 24-12-2012 at 10:33
Say that one has 2 salts: 350g of NaCl (solubility 350 g/L) and 1110g of NaOH (solubility 1110 g/L) in solid state. Let's say that we put them into 1L
of water. Would all of NaOH dissolve? Or would a part of NaOH and part of NaCl dissolve?
I apologize for bad english.
Any help would be appreciated
Vargouille - 24-12-2012 at 13:13
The second one, because of the common ion effect which, short version following, states that if a salt sharing an ion with another salt is added to a
solution of the latter, the one which is less soluble will tend to precipitate. Of course, the dissolution of NaOH increases the temperature of the
solution, so it is possible that both will completely dissolve.
Hexavalent - 24-12-2012 at 13:41
Does dissolution rate also play a part in this? It is a separate property to solubility, and perhaps the salt which has the fastest dissolution rate
will gain the most solubility?
Vargouille - 24-12-2012 at 14:20
I don't think that that would be a significant factor. If NaOH dissolves completely before NaCl, a very small amount of NaCl will dissolve (because
Qsp is less than Ksp) and an amount of NaOH will precipitate to balance out. If NaCl dissolves completely before NaOH, it's not as significant because
NaOH is so much more soluble. As NaOH continues to dissolve, NaCl will continue to precipitate (because Qsp becomes greater than Ksp). In this case,
it is understood that the phenomenon is primarily due to the difference in Ksp.
EDIT: I thought I ought to explain my terms a bit more. "Ksp" is the symbol for the solubility constant of an ionic salt. It defines how
many moles of a salt are dissolved at equilibrium, and is equal to [X]a[Y]b for any salt XaYb. Note that
brackets in chemical equations tend to indicate that the variable is a value of a molar concentration. "Qsp" is a similar value, but can
describe any point outside of equilibrium. It is also calculated in the same way. It is significant because there are three rules when using Qsp and
Ksp: If Qsp < Ksp then the salt will dissolve; if Qsp = Ksp then the system is at equilibrium; if Qsp > Ksp then the salt will precipitate.
[Edited on 24-12-2012 by Vargouille]