Introduction

The pH of acidic, basic, and neutral aqueous solutions of salts is shown with ammonium chloride, sodium acetate, and sodium chloride.

Equations:

NaCl(aq) →  Na+(aq) + Cl-(aq)       

 

 no effect on pH

C2H3O2-(aq) + H2O(l) → HC2H3O2(aq) +OH-(aq)

 (Kb for acetate is 5.6x10-10)   

 basic

NH4+(aq) →  NH3(aq) + H+(aq) 

(Ka for ammonia is 5.6x10-10)

 acidic

Prior to Lecture  (prep time ~ 5 min.)

Assemble the following:

  1. Manufacturer’s packaged bottles of the three chemicals
  2. Universal indicator in a dropper bottle
  3. Three 100 or 150 ml beakers containing equal amounts (very approximately 30 to 40 mL) distilled water
  4. Three scoopulas
  5. Three glass stir rods
  6. Goggles
  7. Reference set of universal indicator in various pHs

Prepare:

0.1M solutions of NaCl (sodium chloride), NaC2H3O2 (sodium acetate), and NH4Cl (ammonium chloride).  Boil 600mL of distilled water for each solution to drive off CO2, cover tightly, and cool.  Weigh out each salt as indicated and dilute to 500mL in a volumetric flask with boiled water.  Keep solutions covered and do not shake.

                                                NaCl                             2.92g

                                                NaC2H3O2                      4.10g

                                                NH4Cl                            2.67g

Transfer 20-25mL of solutions into labeled, screw-top vials.  Bubble nitrogen gas through the solutions to flush out any CO2.  Seal vials and wrap with parafilm.

To Conduct Demonstration:

  1.   Place beakers containing distilled water and universal indicator reference set on the overhead projector.
  2.  Add universal indicator to the beakers of water.
  3. Add a small amount of each salt to a different beaker and stir to dissolve.
  4. Compare the colors to the reference set.

Note:  the NaCl will not be pH 7, but rather approximately pH 5 due to dissolved CO2in the water.  A procedure to prepare the buffers to eliminate the CO2 follows, but was found to be relatively ineffective, so is not used often.

Demo Time: ~5 - 10 minutes

Safety and Disposal

Solutions may be sink disposed..