Lab-molar volume _gases_ by keralaguest


									Honors Chemistry                                                      rev 4/00 Doolittle/Pierson

                         Determination of the Molar Volume of a Real Gas

Pre-lab: The reaction of magnesium metal with a dilute solution of hydrochloric acid is a redox
reaction. It is a single replacement reaction of hydrogen ions by an active metal. The reaction is
The limiting reactant is magnesium metal. The hydrogen gas produced at room temperature will
be collected by water displacement. The volumetric gas collecting tube is calibrated to  0.02
mL. We will collect the following data points; T, P, V and mass of magnesium metal reacted. We
will then calculate the standard molar volume for hydrogen gas.
Some words on procedure:
         a. To collect a gas by water displacement the gas must be insoluble in water and be
              less dense than water. Even meeting these two criteria, there is a third
              consideration. Water evaporates at all temperatures, the concentration of water
              vapor in air or any mixture of gases is a function of temperature. As we collect the
              hydrogen gas, water vapor will saturate the sample. The pressure attributed to water
              vapor is obtained from a table of equilibrium vapor pressure of water, available in the
              lab. To obtain the pressure exerted by the hydrogen alone we will use Dalton’s Law
              of Partial Pressures;

                                  PT = PHydrogen gas + Pwater vapor

             In Dalton’s Law of Partial Pressures there are two important assumptions from the
             model of the Ideal Gas:
                 I. Molecules of any gas occupy a negligible portion of the total volume.
                     Therefore you do not have to account for the volume occupied by the water
                     molecules in your volume measurement of hydrogen gas.
                 II. The molecules in a mixture of gases “behave” in an identical manner. There
                     is no reflection of their chemical makeup in their motions through empty
                     space. This is sometimes stated as “each gas behaves as if it were alone in
                     the empty space”. Once again, we do not have to make any special
                     considerations for the presence of water vapor, except to subtract out its
                     share of the collisions from the value of total pressure exerted by the
                     confined gases.

             In this laboratory we are using a manometer to find the total pressure or P T. You
             need to measure the difference in the height of the water column inside or outside of
             the gas collecting tube, relative to the surface of the water in the beaker. Keeping in
             mind that water is 1/13.6 the density of mercury, write the correct relationship
             between the confined gases and atmospheric pressure. Then solve your equation
             for the pressure of hydrogen alone.

        b. Use only room temperature water so that any gas collected that is below the water
           surface of the beaker is the same temperature as the gas collected in the portion of
           the gas collecting tube that is exposed in air. Read your temperature value to 0.2 C.

        c.   Mass your magnesium ribbon on the analytical balance in the back of the room.

        1. Fill a 1000 mL beaker two thirds with tap water.
        2. Put the buret clamp on the stand.
        3. Fill the gas collecting tube about one-third full with the HCl solution provided. Use
            tap water at room temperature to fill the tube completely. Pour slowly and gently to
            avoid mixing the acid into the tap water.
        4. Meanwhile, your partner will mass the strip of magnesium ribbon and “cage” it with
            copper wire to the bottom of a small cork.
        5. Place the cork loosely in the mouth of the gas collecting tube. Hold it in place while
            you invert the gas collecting tube into the 1000 mL beaker of tap water. Lower the
            gas collecting tube so that the opening with the cork is about ½ inch above the
            bottom of beaker. Use your fingers to loosen the cork completely from the gas
            collecting tube. (The copper cage holding the magnesium should still be positioned
            upright in the gas collecting tube). Clamp the gas collecting tube in place and
            observe the reaction.
        6. When bubbling has ceased, wait a few minutes for temperature equilibrium to re-
            establish, (the reaction is exothermic). Tap the sides of the gas collecting tubes so
            that any small bubbles of hydrogen gas are loosened and join the sample collected in
            the “empty space” above the water in the gas collecting tube. Record the volume of
            the gas.
        7. Measure the temperature of the water bath and record this temperature.
        8. Using a metric ruler, measure the height of the water column in the lab setup, relative
            to the surface of the water in the beaker.
        9. Draw a diagram in your lab book of how the setup looks and record the height value
            on the diagram.
        10. Obtain today’s atmospheric pressure from the classroom barometer.
        11. Record the equilibrium vapor pressure value for water at the temperature recorded
            for the water bath (and the room).

        12. Clean Up. Wash your hands well.

        Setup a data table showing the uncertainties of each measurement.
        Diagram the setup after reaction and label the diagram completely.

       a) Begin with a balanced equation for the reaction;
       b) Calculate the moles of hydrogen formed from grams of Mg. Label this result “n”.
       c) Calculate the pressure (in mmHg) exerted by pure dry hydrogen. Label this “P”.
       d) Convert the pressure of the hydrogen gas to atmosphere units.
       e) Convert the temperature to Kelvin with the correct number of significant figures and
       f) Convert the volume to Liters with the correct number of significant figures and label.
       g) Solve for R.
       h) Check your percent error for R compared to the actual gas constant.
       i) Calculate the standard molar volume of hydrogen at STP.
       j) Calculate your percent error if the accepted value for standard molar volume of an
           ideal gas is 22.41 liters.

Comment on why real gases may deviate from an ideal gas value. (IMPORTANT!)

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