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Determine solubility of boric acid (H3BO3) at various pH's

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This problem indicates that 1 gram of hydrated boric acid (H3BO3) is added to 1 liter of water at pH 7.0 and at standard temperature.

The solubility is stated in the problem as 5.7 grams per 100 ml of solution. A pKa is also given as 9.24.

1) What does the pKa value mean?
2) How would I determine the amount of boric acid that will dissolve at pH 7 given the conditions above?
3) And then how would I determine how much boric acid dissolved would dissolve if the pH were lowered to 6.0?

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Solution Preview

1) pKa = -logKa
where Ka is an acid dissociation constant, which is an equilibrium constant for the dissociation of a weak acid. When an acid, HA, dissolves in water, some molecules of the acid "dissociate" to form hydronium ions and the conjugate base of the acid:
HA = H+ + A-
Thus Ka=[H+][A-]/[HA]

2) You're given solubility of 5.7 g in 100mL. This is the maximum amount of boric acid dissolving in water. Since you adding much less than this threshold (1g in 1L), the equilibrium will need to be ...

Solution Summary

The solution discusses properties of hydrated boric acid.

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See Also This Related BrainMass Solution

Solubility of Zinc Oxide, Boric Acid, and Aluminum Sulfate

Hope you can help....I've always struggled calculating solubilities of compounds at different pH's. I have 3 separate but related questions:

1) 1000 g of zinc oxide is added to a 1 liter container of pH neutral water (pH 7.0); the solubility of the ZnO is given as .29 g/L at pH 7.0; how much more ZnO would dissolve if the pH dropped to 6.0?

2) 1000 g of boric acid H3BO3 is added to a 1 liter solution of pH neutral water; the solubility of the boric acid is given as 57 g/L; again, how much more boric acid would dissolve if the pH dropped to 6.0?

3) Paper-maker's alum (Al2(SO4)3-14H2O has a pH of 3.0; the solution of alum used is given as 38% alum solids; it's solubility is unknown; 1000 g of the alum is then added to 1 liter of pH neutral (7.0) water; a) how much alum dissolves? What is the resultant change to the pH of the overall solution?

I really want to understand how these solubility calcs work, so do your best to make the analysis complete but yet simple.

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