DETERMINATION OF AN EMPIRICAL FORMULA
Name_____________________ Period____
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. Always include the units for values
you measure.
2. Place the beaker on the automatic balance and add 1 piece of zinc. Record the exact mass of
the beaker containing the zinc to the nearest 0.1 g in the data sheet.
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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 –DO NOT
BREATHE THIS DIRECTLY. When most of the zinc has reacted (fizzing has stopped or slowed
down to almost nothing), proceed to the next step.
4. Assemble a ring stand and GENTLY heat your solution (leave about 6 inches between your
Bunsen burner and wire gauze) 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.
IF YOU STILL HAVE A SOLID CHUNK OF ZINC-CAREFULLY ADD MORE HCL
WITH YOUR DISPOSABLE PIPET.
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.
5. Allow your beaker to cool to almost room temperature -- cool to the touch if you were to touch
it. IF YOU STILL HAVE SOLID A SOLID ZINC CHUNK YOU WILL NEED TO REMOVE IT
WITH YOUR FORCEPS.
6. 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.
7. Carry out the calculations described in the data sheet and determine the formula of zinc
chloride.
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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 (go down to #9 to find)
____________
10. Moles chlorine reacted (go down to #10 to find)
____________
11. Ratio (moles chlorine divided by moles zinc)
____________
12. Your formula for zinc chloride
(#11 tells you how many Cl’s you have=subscript)
(The subscript for Zn is 1)
____________
Calculations
9. Show calculation for moles of zinc reacted.
Mass of Zinc (from #3) x
1 Mol Zn
=
Atomic Wt. Of Zn
10. Show calculation for moles of chlorine reacted
Mass of Chlorine (from #7) x
1 Mol Cl
=
Atomic Wt. Of Cl
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