June 16, 2014
Virtual Chem Lab 6-2 & 6-3
Ranking Salt Solutions by pH
6-2: Ranking Salt Solutions by pH
In this assignment you will be asked to rank aqueous solutions of acids, bases, and salts in order of increasing pH. This is most easily done by first identifying the strong acids that have the lowest pH, the strong bases that have the highest pH, and the neutral solutions that have a pH near 7. The weak acids will have a pH between 1 and 6 and the weak bases between 8 and 14. The exact order of weak acids and weak bases is determined by comparing the ionization constants (Ka for the weak acids and Kb for the weak bases). After ranking the pH of these solutions, you will then test your predictions in the laboratory.
1. Arrange the following 0.1 M solutions in order of increasing pH and state why you placed each solution in that position: NaCH3COO, HCl, HCN, NaOH, NH3, NaCN, KNO3, H2SO4, NH4Cl, H2SO3, NaHCO3, Na3PO4 and CH3COOH.
In order of increasing pH:
H2SO4: This is because the first hydrogen is strong and completely ionizes and the second is weak and ionizes very minutely. It is also a lower pH than HCl because the second hydrogen has a smaller amount of of ionization. HCl: Strong acid with 100% complete ionization
H2SO3: Weak acid with Ka1 = 1.7 10-2
CH3COOH: Weak acid with Ka = 1.8 10-5
HCN: Weak acid with Ka = 5.8 10-10
NH4Cl: Ammonium ion - weak acid with Ka1 = 5.6 10-10
KNO3: Neutral salt
NaCH3COO: Acetate ion is a weak base with Kb = 5.6 10-10
NaHCO3: Hydrogen carbonate which is a weak base with Kb = 2.3 10-8
NaCN: Cyanide ion is a weak base with Kb = 1.7 10-5
NH3: Weak base with Kb = 1.8 10-5
Na3PO4: Phosphate ion is a weak base with Kb = 2.2 10-2
NaOH: Strong base with 100% complete ionization.
Once you have predicted the nature of