A1258149944zUohP

1
Gravimetric Determination of Ca
Marble is just CaCO3
GOAL OF EXPERIMENT To determine the calcium
in antacid tablets (CaCO3) METHOD Gravimetric
Analysis Isolate calcium as calcium oxalate
precipitate and compare mass of calcium in
precipitate (known formula CaC2O4H2O) with
original mass of sample.
2

• SCHEDULE
• Today
• Demonstration of cleaning crucibles
• Students check-out materials from stockroom and
  clean crucibles and place in oven
• Demonstration of reaction and filtering
  procedures
• Obtain weight of clean, dry, cool crucibles
• Prepare Samples
• Grind antacid tablet
• Dissolve samples in acid
• Add ammonium oxalate precipitating agent,
  indicator, and urea
• Digest sample (start by 4 PM)
• Next Week
• Filter sample
• Dry product
• Weigh resulting precipitate
• Clean Crucibles

3
Working with partners
Preparation of Crucibles
Drying Procedure
Handling of Desiccators
Transfer of Hot Crucibles to Beaker Return after
Crucibles in Oven
4
The Chemistry of this experiment
dissolution//acid/base
CaCO3 (s) ? Ca2(aq) CO32-(aq) CO32-(aq)
2H(aq) ? H2CO3(aq) ? CO2(g) H2O(l)
dissolution//acid/base
base
oxalic acid
Urea
enzyme
hydrolysis
Very slow reaction imperceptible at room
temperature and neutral pH What do we do to
increase rate? Heat and go to acidic conditions
(low pH) What does your body do? enzyme
increases rate by 1014 !!!!
oxalate
acid/base
Slow production of NH3 makes reaction slow so ppt
is slow in order to make good crystals!!!
precipitation
5

• PROCEDURE
• Obtain mass of clean, dry crucibles
• Grind tablet (lab assistant checks out mortar
  pestal(
• Weigh out samples (0.35-0.38 g) to nearest 0.1
  mg
• Dissolve in 100 mL H2O, 6 mL 6M HCl, adjust pH
  if necessary
• Add pH indicator (20 drops)
• Add (NH4)2C2O4 precipitating agent (20 mL
  sat. sol. 1 mL 6 M HCl)
• Add 15 g urea (use top-loading balance to
  mass)
• Heat at near boiling until color changes to
  yellow and precipitate forms
• Two samples simultaneously, store beakers
  covered with watch glass
• and store crucibles in beaker covered with
  watch glass
• Return CRUCIBLE HOLDER to stockroom
• 
  
• Week 2 to be discussed next lab period
• Filter hot solutions into crucibles (careful
  using paper towel insulator)
• Wash and dry precipitate (water and acetone)
• Weigh precipitate
• Clean Crucibles and put all equipment away in
  proper location
• Do calculations and turn in report