Finding the Formula of a Hydrate from Experimental Data

1. Find the # of g of MN and # of g of H₂O
2. Convert g to mol
3. Divide each "# of mol" by the SMALLEST "# of mol"
4. Use the ratio to find the hydrate's formula

        Example:  

Given the following data

Step 1A)  determine the mass of hydrate (MN^. ? H₂O)

                beaker  +  hydrate                53.84 g

        -      beaker                                    47.28 g

                MN^. ? H₂O  hydrate                 6.56 g

Step 1B) determine the mass of anhydrous salt (MN)

                beaker  +  anhydrous salt     51.84 g

       -      beaker                                    47.28 g

                MN             anhydrous salt      4.56 g

Step 1C) determine the mass of water 

                hydrate                                      6.56 g

       -      anhydrous salt                            4.56 g

                H₂O           water                      2.00 g

 

Step 2A) convert grams to moles

                x mol MN  =  4.56 g MN (1 mol MN / 128 g/mol MN) 

                x  =  0.0356 mol MN

Step 2B) convert grams to moles

                x mol H₂O  =  2.00 g H₂O  (1 mol H₂O / 18 g/mol H₂O) 

                x  =  0.111 mol H₂O

Step 3) divide by SMALLEST "# of moles"

                0.0356 mol MN / 0.0356 mol  =  1 MN

                0.111 mol H₂O / 0.0356 mol  =  3.12 H₂O

Step 4) use the ratio to find the hydrate's formula

                  1 MN ^. 3.12 H₂O   (recall, it is not possible to have 3.12 molecules of water)

                   1 MN ^. 3H₂O        Final Answer

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