Solubility Rules FROM THE UNIVERSITY OF SOUTH CAROLINA

Solubility Rules FROM THE UNIVERSITY OF SOUTH CAROLINA STEPHEN L. MORGAN HTTP: //WWW. CHEM. SC. EDU/FACULTY/MORGAN/RESOURCES/SOLUBILITY/

Rule 1. All compounds of Group IA elements (the alkali metals) are soluble. For example, Na. NO 3, KCl, and Li. OH are all soluble compounds. This means that an aqueous solution of KCl really contains the predominant species K+ and Cl- and, because KCl is soluble, no KCl is present as a solid compound in aqueous solution: KCl(s) => K+(aq. ) + Cl-(aq. )

Rule 2. All ammonium salts (salts of NH 4+) are soluble. For example, NH 4 OH is a soluble compound. Molecules of NH 4 OH completely dissociate to give ions of NH 4+ and OH- in aqueous solution.

Rule 3. All nitrate (NO 3 -), chlorate (Cl. O 3 -), perchlorate (Cl. O 4 -), and acetate (CH 3 COO- or C 2 H 3 O 2 -, sometimes abbreviated as Oac-) salts are soluble. For example, KNO 3 would be classified as completely soluble by rules 1 and 3. Thus, KNO 3 could be expected to dissociate completely in aqueous solution into K+ and NO 3 - ions: KNO 3 => K+(aq. ) + NO 3(aq. )

Rule 4. All chloride (Cl-), bromide (Br-), and iodide (I-) salts are soluble except for those of Ag+, Pb 2+, and Hg 22+. For example, Ag. Cl is a classic insoluble chloride salt: Ag. Cl(s) <=> Ag+(aq. ) + Cl-(aq. ) (Ksp = 1. 8 x 10 -10). Note: A substance's solubility product, Ksp, is the mathematical product of its dissolved ion concentrations raised to the power of their stoichiometric coefficients. Solubility products are relevant when a sparingly soluble ionic compound releases ions into solution. A solubility product is a specific form of an equilibrium constant. Solubility products change with temperature, so the temperature at which a solubility product was measured must always be quoted.

Rule 5. All sulfate ( SO 4=) compounds are soluble except those of Ba 2+, Sr 2+, Ca 2+, Pb 2+, Hg 22+, and Hg 2+, Ca 2+ and Ag+ sulfates are only moderately soluble. For example, Ba. SO 4 is insoluble (only soluble to a very small extent): Ba. SO 4(s) <=> Ba 2+(aq. ) + SO 42 -(aq. ) (Ksp = 1. 1 x 10 -10). Na 2 SO 4 is completely soluble: Na 2 SO 4(s) => 2 Na+(aq. ) + SO 42 -(aq. ).

Rule 6. All hydroxide (OH-) compounds are insoluble except those of Group I-A (alkali metals) and Ba 2+, Ca 2+, and Sr 2+. For example, Mg(OH)2 is insoluble (Ksp = 7. 1 x 10 -12). Na. OH and Ba(OH)2 are soluble, completely dissociating in aqueous solution: Na. OH(s) => Na+(aq. ) + OH-(aq. ), a strong base Ba(OH)2(s) => Ba 2+(aq. ) + 2 OH-(aq. ) (Ksp = 3 x 10 -4)

Rule 7. All sulfide (S 2 -) compounds are insoluble except those of Groups I-A and II-A (alkali metals and alkali earths). For example, Na 2 S(s) <=> 2 Na+(aq. ) + S 2 -(aq. ) Mn. S is insoluble (Ksp = 3 x 10 -11).

Rule 8. All sulfites (SO 3=), carbonates (CO 3=), chromates (Cr. O 4=), and phosphates (PO 43 -) areinsoluble except for those of NH 4+ and Group I-A (alkali metals)(see rules 1 and 2). For example, calcite, Ca. CO 3(s) <=> Ca 2+(aq. ) + CO 3=(aq. ) (Ksp = 4. 5 x 10 -9).

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