; Experiment 5a - Water of Hydration
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Experiment 5a - Water of Hydration

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									                             LAREDO COMMUNITY COLLEGE
     Science Department                    CHEM 1411 Laboratory                                                 1

                             CHEMICAL REACTIONS: Water of Hydration

A.     Short Overview

       Many common substances exist as solid ionic salts that contain water molecules embedded in thier
       crystal structures. This water is referred to as water of hydration, and can be removed from the crystals
       by heating, or dehydration. Such water commonly gives the hydrated salts different properties, such as
       color, than the anhydrous form. This is especially obvious when the metal cation is a member of the
       transition metals.

       In the first of four quantitative experiments involving chemical reactions (water of hydration, empirical
       formulas, limiting reagents, and heat of reaction) that you will perform this semester, you will determine
       the percent water present in an ionic salt. While this is an experiment some of you may have already done
       in a high school chemistry class, it is a valuable experiment because it reinforces the concept of percent
       composition.

       In this experiment you will be provided with a sample of CuSO4•5H2O (used in plant food),
       MgSO4•7H2O (Epsom salts), or FeSO4•7H2O, and asked to determine the experimental % water and the
       actual composition of the solid based upon the amount of water removed from the sample. Each sample
       can be identified by its color.

       The chemical equations for the reactions are

                       CuSO4•5H2O (s)                 CuSO4 (s) + 5 H2O (g)

                       FeSO4•7H2O (s)                 FeSO4 (s) + 7 H2O (g)

                       MgSO4•7H2O (s)                 MgSO4 (s) + 7 H2O (g)

                       MnCl2•4H2O (s)                 MnCl2 (s) + 4 H2O (g)



B.     Procedure

Exercise 1.    Percent Composition of a Hydrated Salt


       Chemicals:     Hydrated Salts (CuSO4•5H2O (blue), FeSO4•7H2O (green), MgSO4•7H2O (white),
                      MnCl2•4H2O (pink)), Propane (C3H8)
       Apparatus:     electronic balances, crucibles, crucible tongs, scoop spatula, tripods, triangles, wire
                      gauze, bunsen burner
       Safety Equipment: goggles, gloves, hood.


       Objectives:     In this experiment you will learn how to:
               1.      heat a sample in a crucible with a Bunsen burner.
               2.      use the method of “weight-by-difference” to determine mass quantities.
               3.      convert grams to moles.
CHEM 1411 Lab                               Water of Hydration                                              2

           4.      determine the percent of water lost by heating a hydrated sample.


   Your instructor will assign you a hydrated salt. Each student will heat one sample. On the data sheet, the
   data for your sample is placed in column listed “Trial 1”. Your partner’s data is listed in column(s) “Trial
   2” (and “Trial 3” if a group of three).

   Use the same electronic balance for each weighing. Tare the balance each time to 0.000 g before
   weighing the evaporating dish.

   CAUTION:        Tie your hair behind your head.

   1.      Obtain a Bunsen burner and tripod from the front of the lab. Place a wire triangle from your desk
           on top of the tripod.

   2.      Clean your crucible & cover with soap and water, rinse with distilled water, and dry the items
           with paper towel.

   3.      Use your crucible tongs to place the crucible and cover on the triangle. (Your instructor will
           demonstrate this process for you.)

   4.      Connect the Bunsen burner to the propane gas line with rubber hose. (CAUTION: Check the
           burner hoses carefully before lighting. Replace the burner hoses if they are old or cracked.)
           Turn on the gas and light the burner. Adjust the flame so that it is pale blue and does not appear
           yellow. (Yellow flames are caused by incomplete combustion and will leave soot on the crucible;
           yellow flames are also cooler than blue flames.) Place the burner under the crucible.

   5.      Heat the crucible until it is red-hot on the bottom for about 5 minutes. Use the tongs to place the
           crucible and cover on the desk, and allow it to cool to room temperature. (Caution: The crucible
           (and cover) will get very hot when heated. Do not touch the items with bare hands.) Allow the
           crucible and cover to cool to room temperatures before weighing. The items are cool enough to
           weigh when they feel cool as you pass your hand over them.

           Do not try to cool down the crucible or cover by running cold water over them; the porcelain
           material will crack. Also, do not set the hot items on your lab notebooks; they will burn through
           the notebook paper.

   6.      Weigh the cool crucible, with lid, on the electronic balance to the nearest 0.001 g. Then use a
           spatula to add 2.5 – 3.0 grams of the assigned hydrated salt to the dish. Reweigh the crucible and
           lid to ± 0.001 gram.

   7.      Replace the wire triangle with the wire gauze and place the crucible and lid on the wire gauze.
           Heat the crucible gently for five minutes by placing the burner under the wire gauze.

   8.      Replace the wire gauze now with the wire triangle and heat the crucible strongly at red heat for
           an additional five minutes. Cool the crucible and lid to room temperature and weigh the items.
           They should weigh less than they did in step 7 due to the loss of water.

   9.      Reheat the crucible and lid on the wire triangle with a strong flame for another five minutes, cool
           and reweigh. The weight after the two heating/cooling increments should agree within 0.005 g. If
     CHEM 1411 Lab                              Water of Hydration                                            3

                not, repeat the heating process for three additional minutes, followed by cooling and weighing.


        10.     Clean the crucible and lid by dissolving the solid in tap water. Rinse the solution in the sink.
                Repeat the cleaning process of the crucible and lid with soap and tap water. At the end rinse the
                dish and lid with distilled water. Dry the crucible and lid with paper towel, and return them to
                your desk. Return all borrowed equipment to the equipment cart at the front of the lab.


        CAUTION: Be sure to turn the propane gas off when you finish.


D.      Disposal

        There are no special disposal instructions for the experiment. All solutions may be discarded in the sink.


E.      Calculations

        1.      Determine the moles of salt in the sample by dividing the mass of the dehydrated salt by the
                formula weight.
        2.      Determine the moles of water by dividing the mass of water lost by 18.015.
        3.      Determine the percent water in the sample by dividing the mass of water lost by the mass of the
                hydrated salt and multiplying by 100%.


E.      Lab Report

        Spreadsheet

        Open the spreadsheet "Water of Hydration" on the CHEM 1411 website. The URL for the website is
        http://www.laredo.edu/science. When you open the website, click on the link for "CHEM 1411".
        You will then click on the icon for the spreadsheet.

        Enter your data in the cells WITH BORDERS only. As you enter your values, the program will
        automatically calculate the masses, moles, and % composition. When you finish, print out the data
        and include in your lab report.

        A sample spreadsheet is provided on the next page.


        Questions

        1.      Determine the experimental % H2O for your samples.

        2.      Determine the % H2O for the following empirical formulas:         CuSO4•5H2O, FeSO4•7H2O,
                MgSO4•7H2O, and MnCl2•4H2O.

        3.      It is possible that the experimental % H2O (question 1) may be significantly lower than the
                calculated theoretical % H2O (question 2) for one of your samples. Provide a possible
                explanation for this.
  CHEM 1411 Lab                          Water of Hydration                                          4




                                                   Trial 1         Trial 2        Trial 3

Unknown number                                        18             48             27


Mass of empty crucible & cover                         18.245         21.882         18.625         g
Mass of crucible, cover, and hydrated
salt                                                   19.965         23.954         20.593         g
Mass of hydrated salt                                      1.720          2.072          1.968      g



Mass of crucible, cover, & dehydrated
salt, final heating                                    19.324         22.907         19.770         g
Mass of dehydrated salt                                    1.079          1.025          1.145      g

Mass of water lost                                       0.641          1.047          0.823        g
Moles of water lost                                  0.035581       0.058118       0.045684       mol

Formula of dehydrated salt                          CuSO4          MgSO4          FeSO4
Molar mass of dehydrated salt                            159.6        120.37         151.94      g/mol
Moles of dehydrated salt                              0.00676        0.00852        0.00754        mol

Moles water ÷ Moles dehydrated salt                         5.26           6.83           6.06

% water (experimental)                                  37.3%          50.5%          41.8%         %
% water in hydrated salt (theoretical)                  36.1%          51.2%          45.4%
% error                                                  3.3%           1.2%           7.8%         %
                             LAREDO COMMUNITY COLLEGE
      Science Department                  CHEM 1411 Laboratory                                          5



                                          Water of Hydration
                                                 DATA SHEET


Name:                                                                    Date
Partner(s)                                                               Section



                                                   Trial 1                  Trial 2               Trial 3

       Unknown number                        (                   )   (                    )   (                 )
       Color of unknown                      (                   )   (                    )   (                 )

1.     Mass of empty crucible & cover                        g                        g                     g
2.     Mass of crucible, cover, and                          g                        g                     g
       hydrated sample
3.     Mass of sample                                        g                        g                     g

       Mass of crucible & dehydrated
       sample
4.        1st heating                                        g                        g                     g
5.        2nd heating                                        g                        g                     g
6.        3rd heating (if necessary)                         g                        g                     g
7.     Mass of dehydrated sample                             g                        g                     g

8.     Formula of dehydrated salt            (              )        (                )       (            )
9.     Molar mass of dehydrated salt                     g/mol                     g/mol                g/mol
10.    Moles of salt                                      mol                       mol                  mol

11.    Mass of water lost                                    g                        g                     g
12.    Moles of water                                        mol                      mol                   mol

13.    Moles water ÷ Moles salt              ______________          ______________           ______________

14.    Empirical Formula, hydrated salt      ______________          ______________           ______________

15.    Experimental % H2O                    ______________          ______________           ______________
16.    Theoretical % H2O                     ______________          ______________           ______________
17.    % Error                               ______________          ______________           ______________


Instructor’s Approval


Show all calculations on the back of the page.

								
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