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Chemistry LibreTexts

Percent Water

  • Page ID
    150656
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    INTRODUCTION

    In an earlier experiment, an unknown crystalline product of an unknown formula was prepared. It is often common for crystalline structures to contain water, and so in this experiment, it will be determined if this unknown is one of the many solid chemicals that are classified as hydrates. The purpose of this experiment is to determine how much if any water is present in the unknown crystals. A hydrate contains water chemically bound in the solid state so that it is present in the compound in stoichiometric amounts.

    The percentage of water of hydration will be determined in this experiment for the unknown crystals by heating a weighed sample of the green crystals in an open container in an oven until all of the water of hydration has been driven off.

    %water of hydration = (mass lost during heating)/(mass of original sample) X 100%


    PROCEDURE


    Step 1


    Before beginning this experiment, the crystals should have been allowed to dry at room temperature in your locked drawer, weighed, and stored in a brown bottle. Then, bottle the bottle with the cap. Then, prepare a table in your research notebook for recording the masses that you will be determining on the analytical balance and the data and time that you do the weighings.


    Follow the specific instructions in the lab manual for preparing for the weighing bottles.

     

     


    Step 2


    After preparing the weighing bottles, weigh each weighing bottle on the analytical balance to the nearest 0.0001 g. Record the masses, time and date in the prepared format in your notebook.

    s2 scale pw.png

     


    Step 3


    Place specified amount of the unknown crystals into each weighing bottle. Weigh the weighing bottles containing the green crystals to the nearest 0.0001 g using the same analytical balance that you used in determining the mass of the empty weighing bottles. Record the masses in your laboratory notebook.

    s3 bottles in beakers pw.png

     


    Step 4


    Place the weighing bottles in a 50mL beakers in a 800 mL beaker, cover with a watch glass, and heat for specified time at some temperature to drive off the water of hydration. Do not leave the crystals in the oven for more than specified time, or they may begin to decompose and turn brown.

    s4 PercentWater1 pw.jpg

     


    Step 5


    After cooling the samples in the desiccator for at least 30 minutes, again weigh each weighing bottle plus sample to the nearest 0.0001 g using the same analytical balance that you used prior to heating the crystals. Record in notebook.

    s5 Sample Dessicator pw.jpg

     


    Step 6


    Return the weighing bottles to the oven for an additional specified amount of heating, followed by cooling, and weighing to ascertain whether more water has been driven off. Record this in your notebook. Repeat this procedure until weighing bottles plus contents have a constant mass (masses agree within specified range).

    s6 PercentWaterGraph pw.jpg

     


    Step 7


    Calculate the loss in mass upon heating for each sample and attribute the mass loss to the water of hydration. Determine the percent water of hydration for each sample, and compute the average. After the final weighing, place the covers on the weighing bottles, and store them with their contents in your drawer.

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