Lab #5   Formula of a Hydrate
 Purpose: To determine the percent by weight of water hydrated to a salt.  To establish for formula of a hydrated salt. Principles: Many salts occurring in nature or purchased from chemical suppliers are hydrated; that is, a number of water molecules are bound to the ions in the crystalline structure of the salt. The number of moles of water per mole of a particular hydrated salt is usually a constant.  For example, ferric chloride is purchased as FeCl3.6H2O, not as FeCl3; cupric sulphate as CuSO4.5H2O, not as CuSO4.  For some salts, heat removes these water molecules: Na2CO3.10H2O  -------->  Na2CO3  +  10 H2O whereas in others, they cannot be removed, no matter how intense the heat, e.g., FeCl3.6H2O. In sodium carbonate decahydrate, 10 moles of water molecules are bound to each mole of Na2CO3 or 180.20 g of H2O per 105.99 g of Na2CO3.  The percent H2O in the hydrated salt is 180.20 g             X  100%  =  62.97% H2O                              180.20 g  + 105.99 g This experiment determines the percent by weight of water in a hydrated salt and its formula. Procedure: Completion of at least 3 trials is suggested for this experiment. To use your time most efficiently, obtain 3 crucibles and lids, identify each crucible and lid as a matched pair, and simultaneously perform the experiment in triplicate.  While one crucible is cooling, another sample can be heated. 1. Support a clean crucible and lid on a clay triangle and heat with an intense flame for 5 minutes. Allow it to cool.  If the crucible is dirty, add a few millilitres of 6 M HNO3.  (CAUTION: Avoid skin contact, flush immediately with water) and evaporate to dryness.   Weight the fired crucible and lid. Handle the crucible and lid with the crucible tongs for the rest of the experiment; do not use your fingers. 2. Add at most 3 g of an unknown hydrate to the crucible and weight it, the lid, and the sample. 3. Return the crucible with the sample to the clay triangle and set the lid off the crucible's edge to allow evolved gases to escape. 4. At first, heat the sample slowly and then gradually intensify the heat.   Do not allow the crucible to become red hot.  This could cause the anhydrous (dried) salt to decompose.  Heat the sample for 15 minutes. Cover the crucible with the lid, cool to room temperature, and weight it, the lid, and the sample. 5. Reheat the sample for 5 minutes.  Reweigh it.  If the second weighing disagrees by anything over 2% with the first, repeat the heating until a constant weight is achieved.
 Prelab Homework 1. A hydrated MgSO4 salt, weighing 3.211 g, is heated in a crucible until reaching a constant weight.  The weight of the anhydrous MgSO4 is 1.570 grams. a)  Calculate the percent of water in the hydrated MgSO4 salt. ____________ % H2O b)   Calculate the moles of H2O removed and the moles of anhydrous MgSO4 remaining in the crucible. ____________ mol H2O                                                                                                        ____________ mol MgSO4 c)   What is the formula for the hydrated MgSO4?                           ____________ 2.   Anhydrous CaCl2 is used as a dessicant in dessicators, i.e. it removes water from the air within the dessicator and forms a hydrated salt.  A 16.43 g mass of anhydrous CaCl2 weighed 21.75 grams after being left in a dessicator for several months.  What is the formula of the hydrated CaCl2 salt? CaCl2.   H2O 3.   In today's experiment what error in the data is likely to occur if the hydrated salt is heated too strongly? 4.    How long should the hydrated salt be heated in removing the water? Data Sheet - FORMULA of a HYDRATE Name of the salt ___________________________ Trial 1               Trial 2                  Trial 3 1.  Mass of crucible and lid                ___________   _____________   ____________ 2.  Mass of crucible, lid and      hydrated salt                                 ___________   _____________   ____________ 3.  Mass of crucible, lid, and      anhydrous salt      1st weighing                                  ___________   _____________   ____________      2nd weighing                                 ___________   _____________   ____________      3rd weighing                                  ___________   _____________   ____________ Calculations: 1.   Mass of hydrated salt                   ___________    ____________    _____________ 2.   Mass of anhydrous salt                 ___________    ____________    _____________ 3.   Moles of anhydrous salt                ___________    ____________    _____________ 4.   Mass of water lost                        ___________    ____________    _____________ 5.   Moles of water lost                       ___________    ____________    _____________ 6.   Percent by weight of volatile       water in this hydrated salt             ___________    ____________    _____________ 7.   Average % H2O in hydrated salt                  ______________________ 8.   Mole ratio of anhydrous salt to       water                                            ___________    ____________    _____________ 9.   Formula of the hydrate                                 ______________________ Questions: 1.    If some volatile impurities are not burned off in Step 1 but are removed in Step 4 is the mass of the anhydrous salt to high or too low?  Explain. 2.   What happens to the sample's reported percent water if the salt decomposes yielding a volatile product?