Stoichiometry – Production of Zinc Chloride
Introduction
In this lab the concept of limiting reactants will be examined. This reaction is an example of a single replacement reaction.
Measuring the mass of each element in a compound and then determining the mole ratio of those elements will indicate
the chemical formula for that compound. The actual mass of zinc chloride collected will be compared to the expected
mass by performing a percent error calculation.
Pre-LAB
1.
Write a balanced equation for the reaction that occurs when zinc is added to hydrochloric acid.
Procedure Day 1 Goggles must be worn at all times during the lab.
Caution – The steam coming out of the top of the test tube WILL BURN.
All groups will use the exact same amount of hydrochloric acid. But each group will use a different quantity of zinc.
1. Carefully measure 10.0 ml of HCl using a small graduated cylinder, and pour into a large test tube labeled with your
assigned number.
2. Determine the mass of all of the zinc you were given (Record your assigned trial number on your lab sheet and label it
on your test tube.) and then drop the zinc into the acid.
3. After making the following observations and answering the questions on your data table, leave the test tube in the test
tube rack.
Observations:
Observe and note any indications that a reaction is occurring.
Collect some of the gas in an inverted test tube and apply the flaming splint test. (Light a wooden splint and hold it in
the mouth of the tube used to collect the gas.)
What does the result of this test indicate about the identity of the gas?
Procedure Day 2 Goggles must be worn at all times during the lab.
Observations
Note which tubes have no zinc left in them.
Note which tubes have zinc remaining in them.
What occurs when the teacher adds more zinc to the sample tube with no zinc left in it?
What can you conclude about limiting reactants for the tubes with no zinc remaining?
What occurs when the teacher add more acid to the second sample tube that had zinc remaining in it.
What can you conclude about the limiting reactant for the tubes with zinc remaining?
4. Mass a clean evaporating dish.
5. Pour ALL the contents of the test tube into an evaporating dish. Rinse the tube with a small amount of deionized water
and be sure to pour the wash water into the dish (beaker) as well.
6. If you have left-over zinc, label a piece of filter paper then record its mass.
7. Remove any Zn remaining with tweezers (If the Zn is broken up and difficult to remove, consult with the teacher) rinsing
the zinc off and catching the rinse water in the evaporating dish, then placing the zinc on the pre-weighed filter paper. At
the end of the lab period when the zinc has dried, weigh the filter paper with the zinc on it.
8. Set up a hot plate to carefully heat the zinc chloride solution. Do not boil too much so that it splashes out (adjust the
temperature of the hot plate as needed). Heat until you see the white crust forming.
Call the teacher over. DO NOT HEAT TOO MUCH. If it starts appearing brownish – STOP and call teacher over.
9. Determine the mass of the dried zinc chloride in its evaporating dish.
Disposal
The remaining zinc (if any) should be returned to Mr. Pauller.
Leave the evaporating dish on the tray on the center lab bench.
Processing the Data –
a. Determine the mass of zinc chloride. (A simple subtraction should take care of this.)
b. Calculate the mass of zinc in your product. (If there was any zinc remaining, a subtraction must be made. If there was
no appreciable amount of zinc remaining assume the starting amount completely reacted.)
c. Calculate the mass of chlorine in your product. (Knowing the mass of zinc chloride produced, make a subtraction to
determine the mass of chloride combined with the amount of zinc reacted.)
More Processing the Data - Three different ways to analyze the results
I. Stoichiometry Calculations
d. Calculate the amount of zinc chloride your group should have been able to produce provided the mass of zinc you
started with.
e. Calculate the percent yield using the following formula.
%Yield =
Actual
x100
Expected
Questions to Ponder
1 A limiting reactant is one that runs out during a reaction, and causes the reaction to stop. In which of the trials was the
zinc the limiting reactant? (Explain how the lab observations support this.)
2 In which trials was the acid the limiting reactant? (Explain how the lab observations support this.)
Error Analysis
5 If the zinc chloride were not completely dried when it was massed, what measurements and calculations would be
affected (higher or lower) and how would it affect the Cl:Zn mass ratio (higher or lower)?
6 If the zinc chloride were heated too long and some of it evaporated, what measurements and calculations would be
affected (higher or lower) , and how would it affect the Cl:Zn mass ratio (higher or lower)?
7 If any zinc pieces left over were still wet when massed, what measurements and calculations would be affected (higher
or lower), and how would it affect the Cl:Zn mass ratio (higher or lower)?