Greetings I'm a bit, if not saying a lot, rusty in this pH exercises.
Imagine you have 1L of a solution of salts dissolved in water which becomes stable(in equilibrium) at pH 7.2.
Can we predict how pH will change if we further add 500mg(let's say) of MgCl2 or MgSO4 to the solution?can we know which of these would vary more the pH of the system?
Does it depend mainly on basicity of Cl- and SO42- or do we need to have in account the initial pH and the salts already dissolved. And would the effect vary if we add the hydrated forms of the salts instead of the anhydrous forms?
My guess is, as Mg2+ is a common ion in both salts, the major effect would come from the counterions in this case Cl- and SO42- with which we would have to enter with Kb of this two ions. Is is as "simple" as that or am I forgetting something?
As you know, salts are can be neutral or acidic or basic. An acidic salt is one which comes from a strong acid & a weak base while a basic salt is one which comes from a weak acid & a strong base.
Both MgCl2 & MgSO4 are considered as acidic salts with the first coming from HCl & Mg(OH)2 "a weak base that is called Milk of Magnesia and is used as stomach antacid" & the second comes from H2SO4 & Mg(OH)2. When a salt is acidic then its pH will be lower than 7. Since sulfuric acid is stronger than hydrochloric acid, then MgSO4 will have a lower pH than MgCl2 but the difference is not large.
For this system, looking at Ka is more appropriate than looking at Kb.
All the above discussion assumes the use of anhydrous salts that go into solution & not hydrated salts since the water of hydration will affect the pH values.
Dear Denis, your question is a bit generic. When you say you have 1L solution of salts you have to be more specific: is there any buffer? Buffered solutions are stable to the addition of small amounts of Acid or Basic compound depending on the concentration of the buffer.
Regarding the pH, the pH of a solution will change if the H+ concentration changes (pH=-log [H+]): adding MgCl2 or MgSO4 will not change the pH because both HCl and H2SO4 (the conjugated acids) are strong acids and are completely dissociated in solution, hence no change in the proton concentration. The following reactions do not occour in water solution:
SO4-- + H2O = HSO4- + OH- and Cl- + H2O = HCl + OH-
I think that I need to elaborate more about the subject. Assume that one prepared aqueous solutions of NaCl, NaHCO3, and NaHSO4 & that their concentrations were all equal. Measurement of their pH values will show that the pH of the first is 7 while the pH of the second is above 7 and the pH of the third is below 7 (Which means that sodium chloride is neutral salt, sodium bicarbonate is basic salt, and sodium bisulfate is acidic salt). The conjugate of the basic salt is an acid “which is HCO3- “and the conjugate of the acidic salt is a base “which is HSO4- “ but these are relative descriptions.
While the conjugate base of H2SO4 is HSO4-, the conjugate base of the acidic salt MgSO4 is SO42- despite the low solubility of this salt in water. The conjugate base for both of HCl & MgCl2 is the chloride Cl-. Both magnesium sulfate & magnesium chloride are acidic salts with pH values lower than 7 & the sulfate has a lower value than the chloride provided that they have the same concentrations.
Thank you for the Answers helped me figure out some points I had forget, thank you both
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