VI.Applications of Solubility 1.Chloride Ion Titration The concentration of chloride ion in a water sample is determined. Adding Ag + to the water sample will cause AgCl (s) to precipitate: Ag + (aq) + Cl - (aq)  AgCl (s) Enough Ag + is added to react with all of the Cl - The moles of Ag + added equal the moles of Cl - that were present in the water sample and [Cl - ] can be calculated.

eg:25.0mL of a water sample was titrated with 0.500M AgNO 3(aq), using sodium chromate as an indicator. At the equivalence point 26.8mL of AgNO 3 solution had been added. What was the [Cl - ] in the water sample? Moles of AgNO 3 used: L = mol of AgNO 3 used. L mol Ag + (aq) + Cl - (aq)  AgCl (s)  moles of Cl - = moles of AgNO 3 used: [Cl - ] = mol = M L

2.Softening “Hard Water” Hard water is caused by the presence of metal ions such as: Ca 2+ and Mg 2+ These ions react with soap molecules and inhibit their function. Slow-moving ground water (such as well water) is most likely to be hard (more time for ions to dissolve into the water). To “soften” water, washing soda may be added (Na 2 CO 3 ) The washing soda precipitates the metal ions: Ca 2+ (aq) + NO (aq)  CaNO 3(s) Mg 2+ (aq) + NO (aq)  MgNO 3(s)