Percent Strontium Chloride Hexahydrate in a Mixture

Introduction

Identification of elements or compounds is called qualitative analysis.  Determination of the AMOUNT of a substance present in a sample is called quantitative analysis.  Methods of quantitative analysis include gravimetric (weighing), volumetric (titration) and instrumental.  Whatever the method the final calculation is the same:

mass  % A = (mass of A determined/sample mass)× 100%

In a gravimetric method determination of component "A" is based on the mass of some product formed on treating a weighed sample containing component "A" and other ingredients.

If a PURE sample of hydrate (salt • nH₂O) is heated above its decomposition temperature the mass loss (difference in the masses before and after heating) gives the mass of water originally hydrated in the salt.  If the identity of the anhydrous salt is known it is possible to evaluate "n" by finding the moles of anhydrous salt, the moles of water and calculating the ratio of moles of water to moles of anhydrous salt:

n = moles of H₂O/moles of anhydrous salt

In this specific experiment we already know the value of n (n=6 for SrCl₂ •6H₂O). However, the strontium chloride hexahydrate (n=6) is mixed with inert substances in the sample. Inert substances are substances that do not decompose or react in any way when the sample is heated according to the directions given. By experimentally find the mass loss upon complete dissociation, we can figure out the percentage of the hydrate, SrCl₂ •6H₂O, in the mixture sample by using the same stoichiometric relationship discussed above.

 The mass loss gives the grams of water lost by the strontium chloride hexahydrate when it decomposes.  The chemical equation for the decomposition is:

SrCl₂ •6H₂O --> SrCl₂ + 6H₂O

Thus the amount of SrCl₂ •6H₂O in the sample can be calculated from the number of grams of water formed by converting grams of water to moles of water, converting moles of water to moles of SrCl₂ •6H₂O, and finally converting moles of SrCl₂ •6H₂O to grams of SrCl₂ •6H₂O:

Mass H₂O = Mass loss on thorough heating

Moles H₂O = Mass H₂O/ Molar Mass H₂O

Moles SrCl₂ •6H₂O = Moles H₂O × (1 Mole SrCl₂ •6H₂O/ 6 Moles H₂O)

Mass SrCl₂ •6H₂O = Moles SrCl₂ •6H₂O × Molar Mass SrCl₂ •6H₂O

% SrCl₂ •6H₂O = (Mass.SrCl₂ •6H₂O/sample Mass) × l00%

BEFORE COMING TO LAB USE THE FOLLOWING SAMPLE SET OF EXPERIMENTAL DATA TO CALCULATE THE PERCENT STRONTIUM CHLORIDE HEXAHYDRATE IN THE SAMPLE.  Check your answer with the instructor on the day you do your lab.

SAMPLE DATA

Procedure

1. Do ONE trial CAREFULLY.  Calculate your percent SrCl₂ •6H₂O and check your answer with the instructor.  If your answer is too far from the correct value you will be asked to do another trial.
2. You must use a clean crucible in this experiment (if the "junk" on the crucible volatilizes during the heating process your results will be worthless).  CHECK THE CRUCIBLE CAREFULLY FOR CRACKS BEFORE YOU START THE EXPERIMENT.  If it is cracked get a new one from the stockroom.
3. Handle the crucible only with tongs from this point on!  (Fingerprints will change the mass of the crucible and when a crucible is hot you don't want to touch it with your hands).  Wrap the tongs around the outside of the crucible.  Don't put one of the tongs inside the crucible (you will increase the chance of contamination AND the chance of dropping the crucible).  You may carry crucible and cover in a clean porcelain evaporating dish.  
4. The crucible must be thoroughly dried BEFORE putting the sample in it.  To dry it set up an iron ring, place a clay triangle over the iron ring, then support the crucible in the clay triangle.  Place the lid on the crucible leaving a small crack for the water vapor to escape (if you put the lid on tight the escaping water vapor will probably pop the lid off causing it to break).  The clay tubes on the triangle may be bent or broken to make the crucible fit in the triangle. 
5. Heat the crucible and its cover strongly (with the bottom of the crucible near glowing red) for at least five minutes. 
6. Remove the burner and let the crucible cool.  After a few minutes it may be transferred to an asbestos pad if desired but DO NOT SET A HOT CRUCIBLE ON THE DESK TOP (yes it will cool faster but it will probably crack and you will have to start over.  Also, considering the small size, crucibles are very expensive). 
7. WHEN THE CRUCIBLE IS COOLED TO ROOM TEMPERATURE weigh it on the Mettler Balance to the nearest milligram (0.00l g).  Remove the crucible from the balance. 
8. Add your unknown sample to the crucible.  Remembering to use tongs, place the crucible back on the balance and weigh it again.  Record all of your weighings DIRECTLY on the report form data sheet. 
9. Place the crucible (using tongs) back on the clay triangle.  Begin with gentle heating then heat more strongly.  The bottom of the crucible should be allowed to near glowing red for ten minutes.  Remove the burner and allow the crucible to cool as before. 
10. When it has cooled to room temperature use tongs to handle it while you take it to the balance room.  Weigh and record the mass.
11. Heat the crucible a second time for about five minutes, cool and weigh.  If the two masses agree to plus or minus five to ten milligrams the decomposition is complete and you may calculate your results.  If the two weighings differ by more than plus or minus five to ten milligrams you must heat strongly for five more minutes, cool and reweigh.
12. Calculate your percent strontium chloride hexahydrate and check your answer with the instructor.  Be sure you use the correct formulas in your calculation.  Use atomic masses accurate to two decimal places from the periodic table.  Round your final percent to the number of significant figures justified by the experimental data.  Remember that treatment of significant figures is different in addition and subtraction operations than in multiplication and division operations.  If your result is not close to the true value list possible errors in your experimental procedure to explain your poor result.