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Zinc Chloride by tyndale

VIEWS: 1,375 PAGES: 5

									               DETERMINATION OF AN EMPIRICAL FORMULA


Introduction: When two elements react to form a compound, the ratio of the masses of the
elements is always the same. (Law of Definite Proportions) John Dalton explained this
observation using atomic theory. An element is composed of atoms. As element A reacts with
element B to form a compound, individual atoms are reacting and the ratio of the numbers of atoms
in the compound is always the same. The simplest ratio of atoms in a compound is known as the
empirical formula of the compound.

                               x atoms A + y atoms B ----> AxBy

The object of this experiment is to determine the empirical formula of Zinc chloride formed by
reacting zinc metal with hydrochloric acid. This reaction is shown below, but the equation is not
"balanced". We will balance it when we experimentally determine the best values of x and y for
zinc chloride. In conducting this reaction, solid zinc metal is placed in a solution of hydrochloric
acid in water. The hydrogen gas produced in the reaction bubbles out of the solution. Zinc
chloride, the second product is a white solid that initially remains dissolved in the water of the
solution. You will evaporate the water to determine the mass of zinc chloride produced.

                          x Zn + y HCl ---------> ZnxCly + y/2 H2


Precautions: Wear your goggles at all times. Hydrochloric acid can cause severe burns, especially
to the eyes. Thoroughly clean all spills immediately by diluting with large quantities of water and
wiping dry with paper towels. To avoid confusion between water and hydrochloric acid, keep your
work area dry at all times. Acids cause a stinging sensation - if your hands, arms, or face sting,
rinse immediately with water. Hydrochloric acid also burns holes or causes discoloration in
clothing - wear your lab aprons.


Procedure:

1. Clean and dry a 100 mL beaker. Once you've cleaned and dried it, avoid touching it with bare
hands. Your hands will leave moisture and oils on the beaker and change its weight. Use either a
dry paper towel or crucible tongs to handle your beaker. Weigh the beaker on an automatic
balance. Record this measurement in the data sheet, showing the proper number of significant
figures. Always include the units for values you measure.

2. Place the beaker on the automatic balance and add 0.6-1.2 g of zinc—your instructor will tell
you how much you should add. Record the exact mass of the beaker containing the zinc to the
nearest 0.001 g in the data sheet. (Add zinc slowly using your spatula until you have approximately
the amount assigned to you. If you are assigned a mass of 0.8 g, for example, and you actually use
0.785 g or 0.834 g this will be okay. Record the actual mass of the beaker and zinc with 3 decimal
digits. Eventually you will calculate the mass of zinc used by subtracting the mass of the beaker



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from this value. If you spill zinc in the vicinity of the balance, clean it up carefully without
touching the balance pan. )

3. Pour approximately 15 mL (need not be exact, within + 1 mL is sufficient) of 6 M HCl into a
graduated cylinder. Slowly and carefully add this solution to the beaker containing the zinc in the
hood. When most of the zinc has reacted, proceed to the next step.
4. Assemble a ring stand in the hood and GENTLY heat your solution to complete the reaction
and drive off excess HCl and water. Continue heating the solution to drive off the HCl and water
until the contents become paste-like. Do not allow the contents to spatter by overheating.
Occasionally let the flame travel up the outside of the beaker to evaporate water condensed on the
sides.

    Zinc chloride is a white powder with a melting point of 283o C. If you get the contents of the
beaker too hot in the latter stages of the drying process, you may have molten zinc chloride present.
It will look like a thick syrup. Dense, white fumes will be evolved if you continue to heat the
molten zinc chloride. If you see this occurring, stop heating immediately. When you believe that
all the water has been removed, allow your beaker to cool somewhat. You should see the zinc
chloride appearing as a white solid at the bottom and on the sides of the beaker. Additional heating
may be required if you have not driven off all the water from the sides of the beaker.

6. Take your beaker and ring stand back to your lab bench. CAUTION: Your beaker AND ring
stand are very hot. Be careful. Do burn yourself or drop your beaker during this process.

7. Allow your beaker to cool to almost room temperature -- cool to the touch if you were to touch
it. Reweigh the beaker containing zinc chloride and record your answer in data sheet. Do not wait
too long to reweigh your beaker and products. Zinc chloride will absorb water from the
atmosphere.

8. Carry out the calculations described in the data sheet and determine the formula of zinc chloride.
Be sure to show the correct number of significant figures for all values, measured and
calculated.




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                                        DATA SHEET
Name ____________________________                    Lab Partner ___________________________


1. Mass of beaker                        ____________

2. Mass of beaker and zinc               ____________

3. Mass of zinc                          ____________

4. Mass of beaker and zinc chloride      ____________

5. Mass of zinc chloride                 ____________

7. Mass of chlorine in zinc chloride     ____________
   (Mass zinc chloride - mass zinc)

8. Ratio (mass chlorine/mass zinc)       ____________

9. Moles zinc reacted                    ____________

10. Moles chlorine reacted               ____________

11. Ratio (moles chlorine/moles zinc)    ____________


12. Your formula for zinc chloride       ____________




                                        Calculations

9. Show calculation for moles of zinc reacted.




10. Show calculation for moles of chlorine reacted




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                                          Questions

1. Using the formula for zinc chloride, write the balanced equation for the reaction of zinc with
hydrochloric acid.




2. Assuming and adequate supply of HCl, what mass of zinc chloride would have been formed if
you had used exactly twice as much zinc? (Give a numeric answer based upon the quantity of zinc
you reacted and the quantity of zinc chloride you obtained. Show your calculation.)




3 A student performs an experiment similar to the one we did by reacting a sample of yttrium
metal (Y) with hydrochloric acid to give yttrium chloride. The data are given below.
    Carry out the necessary calculations to obtain the formula of yttrium chloride.
    Show clearly how you made all calculations.
                                                            Part b answers        Part c answers
a) Mass of empty beaker                   50.135 g           50.135 g               50.135 g
b) Mass of beaker and yttrium             51.418 g           51.418 g               51.418 g
c) Mass of yttrium metal                  ________           ________               ________

d) Mass of beaker and yttrium chloride 52.928 g              ________               ________

e) Mass of yttrium chloride               _______            ________               ________

f) Mass of chlorine in yttrium chloride _______              ________               ________

g) Moles yttrium reacted                  _______            ________               ________

h) Moles chlorine reacted                 _______            ________               ________

i) Ratio (moles chlorine/moles yttrium) _______              ________               ________

j) Formula for yttrium chloride           _______            ________               ________

Show all calculations clearly below. Identify the calculation you are making with its letter.




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b) Assume that during the experiment, this student heated too rapidly and spattered exactly 0.050 g
of his product out of the beaker. How would this affect his answer? Recalculate the (incorrect)
formula for yttrium chloride using this data. Show the calculations below and the results in the
spaces provided on the previous page.




Summarize the effect of this type of error. How will the loss of some of your product when you
heat it affect the ratio of Cl/Y you determine?




c) Assume a second student did not heat enough and some water condensed on the sides of the
beaker and was not driven off. If exactly 0.050 g of water condensed on the sides of the beaker as
he was heating, how would this affect his answer? Recalculate the (incorrect) formula for yttrium
chloride using this data. Show the calculations below and the results in the spaces provided on the
previous page.




Summarize the effect of this type of error. How will the failure to drive off all the water when you
heat it affect the ratio of Cl/Y you determine?




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