Name
Date
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Chemistry Unit 5 - Empirical Formula Lab
Introduction
In this experiment, a measured amount of zinc will be allowed to react with hydrochloric acid, HCl. One
of the reaction products will be zinc chloride. You will obtain data that will enable you to determine the
empirical formula of zinc chloride, ZnxCly. Empirical means “based on experimental evidence”.
Pre Lab Instructions*:
1. Read through the following Procedures section and highlight or mark each of the following items using
two different colors. Highlight each item each time it appears in the procedure. Include a key.
Color/mark 1: materials, chemicals
Color/mark 2: data to record (values and observations)
2. Start a new page in your lab notebook, write an appropriate problem statement for the investigation.
3. Beneath your problem statement, generate an appropriate data table for this lab. Include the necessary
elements of a good data table (title, headings, etc.). In the left side of the table, identify each row with
the reaction will be observing during the lab. Remember, the lab involves nine reactions. On the right
side of the table, be sure to leave enough space for the measurements and/or observations you will
actually make and record during the lab. You may wish to review your highlighting to help you with this.
*Pre-Lab elements MUST be completed in order to participate in the lab.
Procedure - Part 1
1. Obtain and label a clean, dry Erlenmeyer flask with your class period and group number. Find the mass of
the labeled flask (to the nearest 0.01g)
2. Add the number of zinc pieces as instructed by your teacher. Find the mass of the flask and zinc to the
nearest 0.01g.
3. Add 50 mL of 3M HCl. Record your observations.
4. Place your labeled flask on one of the hot plates in the fume hood.
Procedure - Part 2
5. Set up a bunsen burner, ring stand, ring and wire screen so you can heat the flask.
6. Zinc chloride absorbs water readily from the air. In order to remove the water, heat the flask and contents
for a minute or two. As long as the contents appear to bubble, water is evaporating. However, when the
contents begin to smoke, stop heating immediately. Remove the flask (use tongs, it's hot) and allow it to
cool on the metal base of the ring stand. Note how the zinc chloride solidifies from the molten state.
7. When the flask is cool enough to handle, find the mass of the flask and zinc chloride. (mass line 1)
8. Repeat steps 6 and 7. (mass line 2)
a. If this second mass is more than 0.02g lighter than the previous mass,
repeat steps 6 and 7 once more. (mass line 3, if needed)
b. If the mass is unchanged, wash out the contents of the flask as instructed by your teacher.
Modeling Chemistry
1
U5 Emp formula lab v2.0
Data
Make a data table in your lab notebook so that you can record the following data as you carry out your
procedures. Be sure to include relevant uncertainty!
Mass of labeled flask
Mass of flask + zinc
Mass of flask + zinc chloride (line 1)
Mass of flask + zinc chloride (line 2)
Mass of flask + zinc chloride (line 3)
g
g
g
g
g
Qualitative Observations – Include beneath data table in your notebook as you work in lab
Calculations
1. Determine the mass of zinc reacted.
2. Determine the mass of zinc chloride produced (which value should you use?).
3. Determine the mass of chlorine in the zinc chloride.
4. Determine the number of moles of zinc, then the number of moles of chlorine.
5. Determine the ratio:
moles Cl
. Express your answer with appropriate labels and to an appropriate
moles Zn
number of significant figures.
Conclusion & Evaluation
Address the following in your lab report using complete thoughts and sentences.
1. Based on your data, what do you think is the likely ratio:
atoms Cl
? Explain.
atoms Zn
2. Based on your data, what is the chemical formula (empirical formula) of zinc chloride? What
information does the chemical formula provide? Why is this referred to as an “empirical” formula?
3. Describe how the value of your un-rounded ratio in calculation 5 compare to the accepted value.
Then evaluate* your method to see what limitations or weaknesses could have led to this result.
*Note: To evaluate means to state a specific limitation, weakness, or mistake in your methods AND to
appreciate the effect on the results. This means you need to state a specific reason AND explain why this
led to your ratio being too high or too low.
4. Suppose that you had not driven off all the water from the zinc chloride. How would this error have
affected the ratio in calculation 5? Show evidence for your prediction by repeating calculations 2 – 5
using the first value for the mass of the flask and zinc chloride. (If the mass never varied, see your
teacher)
Modeling Chemistry
2
U5 Emp formula lab v2.0