Chemistry 2202
Empirical Formula Determination
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Name _____________________
_____________________
_____________________
Purpose
To determine the empirical formula of a compound containing zinc and chlorine
Introduction
When zinc and hydrochloric acid react, one of the products is a compound containing
zinc and chlorine called zinc chloride. The other product is hydrogen gas.
zinc(s) + hydrochloric acid(aq)  zinc chloride(aq) +
hydrogen(g)
The zinc chloride is soluble so you will not be able to see it when the reaction occurs.
However, if you evaporate the water from zinc chloride (aq), it will remain as a white
residue.
In this experiment you will react zinc with hydrochloric acid, and determine how much
zinc actually reacted. You will evaporate the water to find the mass of the white residue
of zinc chloride that was produced during the reaction. From this information you will be
able to calculate the empirical formula of the zinc chloride.
Materials:
2 pieces of zinc
10 mL of 6 M hydrochloric acid
Apparatus:
mass balance
250 mL beaker
funnel
oven
wash bottle
stirstick
filter paper
10 mL graduated cylinder
Cautions!!!
6M Hydrochloric acid:
Toxic
Corrosive
80 or100 mL beaker
grease pencil and lead pencil
evaporation dish
Chemistry 2202
Empirical Formula Determination
If you spill hydrochloric acid on your hands (it will feel tingly, then start to itch) wash
your hands with water immedicately. See your teacher if the itching persists.
Absolutely no contact lenses may be worn for this lab, and lab goggles must be worn at
all times. If you get hydrochloric acid in your eyes, go to the eye wash station and flush
continuously for 15 minutes.
Hydrogen gas:
Flammable gas
Take care when you add the zinc to the hydrochloric acid. The solution will get quite
warm, and hydrogen gas is released. Do not make any sparks or light any bunsun burners
around the mixture - the splattering warm acid mixture could be quite dangerous.
Procedure
Day #1 - The Reaction
1.
Measure 10.0 mL of 6.0 M hydrochloric acid using a 10 mL graduated cylinder.
Pour the acid into a clean 80 or 100 mL beaker. Put a distinguishing mark on
your beaker with a lead pencil so you can identify it next class.
2.
Mass two pieces of zinc. Record mass on next sheet! Add the pieces of zinc to
the beaker that contains the acid from step 1. Record your observations of what is
happening in the beaker.
3.
Leave the reaction overnight so that it can go to completion.
Day #2 - Evaporating the Water
The reaction mixture should have stopped bubbling, and there should be some zinc left in
the beaker that has not reacted. This is because we have used up all or most of the
hydrochloric acid, and the zinc has nothing to react with. We must carefully collect all
the unreacted zinc in order to find out just how much zinc reacted. We will separate the
zinc from the aqueous solution of zinc chloride by filtering.
4.
Label a clean dry 250 mL beaker with a pencil. Determine its mass. Record
mass on data sheet!
5.
Mass a piece of filter paper. Record mass on the data sheet!
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Chemistry 2202
Empirical Formula Determination
6.
Put the filter funnel in the beaker. Flute the filter paper (teacher demo)and put in
the funnel. Carefully filter the mixture from the reaction beaker into the 250mL
beaker, transferring as much zinc as possible.
7.
Wash the reaction beaker 3 times with small portions of deionized water from a
wash bottle. Swirl and pour through the filter. Try to ensure as much zinc is in the
filter paper as possible.
8.
Try to ensure all the aqueous zinc chloride is washed off the filter paper into the
250mL beaker by using your wash bottle to rinse the filter paper 3 times.
Remember that the zinc chloride is dissolved in the mixture, so it is it is invisible.
9.
When the filtering is finished, carefully remove the filter paper from the funnel
and spread out in a marked (grease pencil) evaporation dish. Leave it in a safe
place to dry.
10.
Put your marked 205 mL beaker containing zinc chloride (aq) in the oven to dry.
Day #3: Massing The Zinc Chloride And Unreacted Zinc
11.
Mass the dry filter paper with the unreacted zinc. Record mass on data sheet!
12.
Mass the beaker with the white zinc chloride residue. Record mass on data
sheet!
Results
DataTable
1. Initial mass of zinc
2. mass filter paper
3. mass of empty 250 mL beaker
4. mass filter paper and unreacted zinc
5. mass of 250 mL beaker + dry zinc chloride
Observation of the reaction :
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Chemistry 2202
Empirical Formula Determination
Calculations
1.
From your data, determine the empirical formula for zinc chloride. Show ALL
steps and calculations. (Hint: You probably experienced lots of experimental
error. Due to these errors, the "whole number" ratio you calculate in the last step
may not be as close to a whole number as the examples done in class. Simply
round to the nearest whole number.)
Conclusions
Write a conclusion for your experiment. Compare the empirical formula you determined
to the theoretical empirical formula, and state whether you felt you achieved the purpose
of this lab. Also state at least 4 sources of error.
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Chemistry 2202
Empirical Formula Determination
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