Predicting Solubility with Solubility Rules

Solubility and solution concentrations are usually expressed in units of mol/L

Different compounds have different solubility Generally if a solute has a solubility of greater

than 0.1 M in a particular solvent, it is classified as soluble in that solvent

If it is less than 0.1 M, then the solute is insoluble (forms a precipitate) in that solvent

Through experimentation, a set of qualitative “solubility rules” have been determined

Solubility Rules

Rule No. Rule statement summary

1 Hydrogen, ammonium and group l ions form soluble compounds with all negative ions

2 Acetate and nitrate ions form soluble compounds with all positive ions

Ions of group l (Li+, Na+, K+, etc.), NH4

+ and H+

All compounds soluble

CH3COO-, NO3-

All compounds soluble

3 Sulfide ions form soluble compounds only with ions listed in rule 1 and with group ll cations

4 Chloride, bromide and iodide ions form compounds that have low solubility with silver, lead (ll), mercury (l) copper (l) and thallium positive ions only

S-2 with Mg+2, Ca+2, Sr+2, Ba+2, Ra+2 and ions in group l

Soluble compounds

S-2 with all other positive ions Forms precipitates

Cl-, Br- and I- with Ag+, Pb+2, Cu+1, Tl+ Forms precipitates

5 Sulfate ions forms compounds that have low solubility with calcium, strontium, barium, radim and lead (ll) positive ions

6 Hydroxide ion form compounds that are soluble with those listed in rule 1 and strontium, barium, radium and thallium

SO42- with Ca+2, Sr+2, Ba+2,

Ra+2, Pb+2 Forms precipitates

OH- with Sr+2, Ba+2, Ra+2, Tl+ and ions in rule 1 Soluble compounds

OH- with all other positive ions Forms precipitates

7

Phosphate, carbonate and sulfite ions form compounds that have low solubility with all positive ions except those listed in rule 1

PO4-3, CO3

-2, SO3-2 with positive

ions in rule 1Soluble compounds

PO4-3, CO3

-2, SO3-2 with all other

positive ionsForms precipitates