# Pollution and hard water treatment by precipitation

##### Intros
###### Lessons
1. What is 'pollution' and hard water?
2. Using precipitation to clean water.
3. Worked calculation: Using Ksp and equilibrium to treat water.
4. Hard water.
##### Examples
###### Lessons
1. Use the Ksp expression to calculate concentrations of aqueous ions in treating water pollution.

A sample of industrial waste water has a Pb2+ concentration of 7.9*10-4 M, and 'acceptable levels' of lead ions in water have been stated as a maximum of 7.25*10-10 M.

What amount of OH- ions need to be added to precipitate lead ions, so that [Pb2+] drops to the acceptable level at most? The Ksp for Pb(OH)2 is 1.43*10-20 M.1
1. Use the Ksp expression to calculate concentrations of aqueous ions in treating water pollution.

A sample of industrial waste water has a Pb2+ concentration of 6.5*10-3 M, and 'acceptable levels' of lead ions in water have been stated as a maximum of 7.25*10-10 M.

What amount of CO32- ions need to be added to precipitate lead ions, so that [Pb2+] drops to the acceptable level at most? The Ksp for PbCO3 is 7.4*10-14 M.1

1 Reference for solubility constant data: http://www4.ncsu.edu/~franzen/public_html/CH201/data/Solubility_Product_Constants.pdf