Vocabulary Hydrate - chemically, a salt with water molecule(s) attached MN. xH2O e.g. CaSO4 . ½ H2O Anhydrous - without water MN. xH2O MN + 5H2O D Decrepitation - water rapidly “popping” off a hydrate when heated rapidly Hygroscopic - absorbs moisture readily from its surroundings The desiccator is used to store dried samples in a dry atmosphere. It should not be used to dry an object, but to maintain an already dried object indefinitely in a dry condition. Most desiccants are ionic compounds that form hydrates. The most common used are CaCl2 (forms CaCl2 x 2 H2O) and CaSO4 (forms CaSO4 x 0.5 H2O). Silica gel (SiO2) is also used for some applications. Because desiccants absorb moisture, they should be handled with care, wearing goggles and a lab coat. Desiccator image (top right) www.seedburo.com/online_ cat/categ01/sd200.asp Gypsum (used in drywall) is CaSO4 . 2H2O (calcium sulfate dihydrate) Turquoise (jewelry) is CuAl6(PO4)4(OH)8 . 4H2O (a tetrahydrate) Desiccant - a drying agent used to absorb moisture Desiccator

Hydrate Ionic compounds that contain specific ratios of loosely bound water molecules, called waters of hydration. Waters of hydration can be removed by heating. Compounds that differ only in the numbers of waters of hydration can have very different properties. Copyright 2007 Pearson Benjamin Cummings. All rights reserved.

Step 1A) determine the mass of hydrate (MN. ? H2O) Formula of a Hydrate Given the following data BaCl2.3H2O mass of beaker   47.28 g   beaker mass of beaker and sample before heating 53.84 g beaker  +  MN  +  H2O mass of beaker and sample after heating 51.84 g beaker  +  MN molar mass of anhydrous salt 128 g / mol   A B C D hydrate 6.56 g MN. xH2O Step 1A)  determine the mass of hydrate (MN. ? H2O)                 beaker  +  hydrate                53.84 g         -   beaker                                  47.28 g                 MN. ? H2O  hydrate                 6.56 g

Formula of a Hydrate Given the following data mass of beaker   47.28 g   beaker mass of beaker and sample before heating 53.84 g beaker  +  MN  +  H2O mass of beaker and sample after heating 51.84 g beaker  +  MN molar mass of anhydrous salt 128 g / mol   hydrate 6.56 g MN. xH2O anhydrous salt 4.56 g MN 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 Formula of a Hydrate Given the following data mass of beaker   47.28 g   beaker mass of beaker and sample before heating 53.84 g beaker  +  MN  +  H2O mass of beaker and sample after heating 51.84 g beaker  +  MN molar mass of anhydrous salt 128 g / mol   hydrate 6.56 g MN. xH2O anhydrous salt 4.56 g MN Step 1C) determine the mass of water                   hydrate                                      6.56 g        -      anhydrous salt                            4.56 g                 H2O           water                      2.00 g

Formula of a Hydrate Step 2A) convert grams to moles (salt)                 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 (water)                 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 To determine the empirical formula from the mass percentages of the elements in a compound, the following procedure is used: 1. The mass percentages are converted to relative numbers of atoms, 2. A 100 g sample of the compound, is assumed 3. Each of these masses is divided by the molar mass of the element to determine how many moles of each element are present in the 100 g sample 4. The results give ratios of the various elements in the sample—but whole numbers are needed for the empirical formula, which expresses the relative numbers of atoms in the smallest whole numbers possible 5. To obtain whole numbers, the number of moles of all the elements in the sample are divided by the number of moles of the element present in the lowest relative amount. Results will be the subscripts of the elements in the empirical formula 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

Formula of a Hydrate - Prelab Printable copy of LAB 1) Weigh dry beaker (record mass in data table) 2) Add 1 spoonful of hydrate to beaker (weigh) 3) HEAT: ~10 minutes gently (low heat) ~3 minutes and HIGH ~7 minutes 4) Cool 3 minutes and weigh 5) HEAT: HIGH ~5 – 7 minutes 6) Cool 3 minutes and reweigh 7) Repeat steps #5 and #6 if “constant weight” is not observed DATA TABLE Molar mass of anhydrous salt = 120.3 g Percentage composition of water = 51.2%

Traditional heating of hydrate using a crucible. http://www.chemmybear.com/groves/images/demos/hydrate1.jpg Traditional heating of hydrate using a crucible. I’ve had very good luck using a beaker.

Analysis 1. What is the mass difference between the hydrate and the ionic compound? What does this mass represent? 2. To determine the formula of the hydrate, what piece of information do you need? Ask your teacher for this information. 3. What is the empirical formula of the hydrate? Show your work.

Conclusions 1. Suppose the heating did not remove all of the water from the hydrate. How would this affect your results? 2. List at least two other possible sources of error in this investigation. you need? Explore Further A desiccant is a compound used to absorb moisture from the air. How could the formula of a hydrate indicate its effectiveness as a desiccant?