# Solubility Equilibria Will it all dissolve, and if not, how much?

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Solubility Equilibria Will it all dissolve, and if not, how much?

All dissolving is an equilibrium. If there is not much solid it will all dissolve. As more solid is added the solution will become saturated. Soliddissolved The solid will precipitate as fast as it dissolves. Equilibrium….aka…saturated solution

Watch out Solubility (molar solubility, s) is not the same as solubility product. Solubility product is an equilibrium constant. it doesn’t change except with temperature. Solubility is an equilibrium position for how much can dissolve. A common ion can change this.

Practice: Calculate the K sp The solubility of iron(II) oxalate FeC 2 O 4 is 65.9 mg/L ◦ K sp = x 2 = 4343 The solubility of Li 2 CO 3 is 5.48 g/L ◦ K sp = (2x) 2 x = 4x 3 = 658

Calculating Solubility The solubility is determined by equilibrium. Its an equilibrium problem. Watch the coefficients

Practice: Calculating Solubility Calculate the solubility of SrSO 4, with a Ksp of 3.2 x 10 -7 in M and g/L. ◦ 3.2 x 10 -7 = x 2 ◦ x = 5.7 x 10 -4 Calculate the solubility of Ag 2 CrO 4, with a Ksp of 9.0 x 10 -12 in M and g/L. ◦ 9.0 x 10 -12 = (2x) 2 x = 4x 3 ◦ x = 1.31 x 10 -4

Relative Solubilities: Reminder Ksp will only allow us to compare the solubility of solids that fall apart into the same number of ions. The bigger the Ksp of those the more soluble. If they fall apart into different number of pieces you have to do the math.

Common Ion Effect If we try to dissolve the solid in a solution with either the cation or anion already present less will dissolve.

Common Ion Effect How is the solubility of SrSO 4, with a Ksp of 3.2 x 10 -7 effected by the addition of a solution of 0.010 M Na 2 SO 4 SrSO 4 Sr 2+ + SO 4 2- Na 2 SO 4 2Na + + SO 4 2- Common ion decreases the solubility

Common Ion Effect How is the solubility of SrSO 4, with a Ksp of 3.2 x 10 -7 effected by the addition of a solution of 0.010 M Sr(NO 3 ) 2. SrSO 4 Sr 2+ + SO 4 2- Sr(NO 3 ) 2 Sr 2+ + 2NO 3 - Common ion decreases the solubility

Solubility and pH Mg(OH) 2 solubility is pH dependent. How? In a BASIC solution, the concentration of hydroxide ion in solution is high. ◦ So solubility is LOWER than in pure water. In an ACIDIC solution, we have a significant amount of hydronium, which can react with hydroxide. This lowers the hydroxide concentration and makes ◦ So solubility if higher than in pure water

pH and solubility Dissociate the salt to determine if the solution is acidic, neutral or basic If a compound is BASIC, then it will be LESS SOLUBLE in basic solutions, and MORE SOLUBLE in acidic solutions! If a compound is ACIDIC, then it will be MORE SOLUBLE in basic solutions, and LESS SOLUBLE in acidic solutions! If a compound is NEUTRAL (neither acidic nor basic), then its solubility will be UNAFFECTED by pH

pH and solubility Na 3 PO 4 Compared to water, how is the solubility effected in an acid? in a base? Na 3 PO 4 3Na + + PO 4 3- PO 4 3- + H 2 O HPO 4 2- + OH - wa sb

One More Twist Explain how the solubility of AgCl is affected by adding an ammonia solution. AgCl + NH 3 [Ag(NH 3 ) 2 ] + + Cl - Solubility increases as the silver dissolves to form the complex ion.

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