Determining the Formula of a Compound Lab: Formula of a Chloride
Name __________________________
Introduction
In this experiment we will attempt to determine the formula of the compound zinc
chloride by measuring masses involved in the reaction of zinc with hydrochloric acid.
Safety Notes:
 Wear your safety glasses at all times during this lab.
 Tie back long hair and take off jackets with long sleeves.
 Wash your hands thoroughly before leaving the lab area.
Procedure:
1) Determine the mass of a clean, dry 100 mL beaker to the nearest 0.001 g and record
this mass in the data table.
2) Obtain a small amount of powdered zinc metal, and place it in the beaker. Then
determine the mass of the beaker plus zinc and record in the data table.
3) Using a graduated cylinder to measure, add 10 mL of 3M HCl. Using a hot plate,
gently heat the mixture until all of the zinc has dissolved.
4) Using a Bunsen burner, heat the solution until all of the water and excess HCl have
been boiled away. Stop the heating as the last bit of liquid disappears. Do not
continue heating after it has boiled to dryness as the compound will begin to
decompose.
5) Cool for several minutes and then determine the mass of the beaker and contents.
Data:
mass of beaker
mass of beaker + zinc
mass of beaker + zinc chloride compound
Calculations: (Clearly label all calculations, and pay attention to sig figs.)
1) Determine the mass of zinc used.
2) Determine the mass of chlorine that combined with the zinc.
3) Determine the moles of zinc used.
4) Determine the moles of chlorine that combined with the zinc.
5) Determine the ratio of: moles chlorine/moles zinc by taking the results of
calculation 4 and dividing by the results of calculation 3.
6) Write down the experimentally-determined formula for zinc chloride in the lowest
whole number ratios.
7) The accepted result of calculation 5 would be exactly 2. Determine your percent error
between what you determined for calculation 5, and the accepted value.
Follow-up Question:
Suppose a student made the mistake of not heating the compound long enough to drive
away all the excess HCl. What effect would this error have on the moles Cl/moles Zn
ratio? Explain.
Pre-Lab:
Must be done before beginning the lab. Answers can be found
on the last page. Make sure to show all work and all units!!!
In an experiment to determine the formula of the compound aluminum chloride, a
student obtains the following data:
Mass of beaker:
Mass of beaker + aluminum:
Mass of beaker + aluminum chloride
27.251 g
27.414 g
28.039 g
1) Determine the mass of aluminum used.
2) Determine the mass of chlorine that combined with the aluminum.
3) Determine the moles of aluminum used.
4) Determine the moles of chlorine that combined with the aluminum.
5) Determine the ratio of: moles chlorine/moles aluminum by taking the results of
calculation 4 and dividing by the results of calculation 3.
6) Write down the experimentally-determined formula for aluminum chloride in the
lowest whole number ratios.
7) The accepted result of calculation 5 would be exactly 3. Determine you percent error
between what you got for calculation 5, and the accepted value.
8) In an experiment to determine the formula of the compound niobium chloride, a
student conducts and experiment and obtains the following data:
Mass of beaker:
Mass of beaker + niobium:
Mass of beaker + niobium chloride
34.091 g
35.206 g
37.333 g
What is the formula for niobium chloride according to this student’s data?
1. 0.163 g
2. 0.625 g
3. 0.00604 mol
4. 0.0176 mol
5. ratio = 2.92 (make sure to use unrounded numbers when dividing!)
6. AlCl3
7. % Error = 3 %
8. NbCl5
Hypothetical Data (to be used if absent on the day of the lab):
mass of beaker
mass of beaker + zinc
mass of beaker + zinc chloride compound
54.089 g
54.658 g
55.222 g