LAB #6.2: Determination of the Empirical Formula of Magnesium Chloride
Background: You have observed the reactions of different metals with hydrochloric acid. You will make careful measurements in this lab investigation to determine the formula for one of those compounds- magnesium chloride.
Purpose:
To determine the empirical formula of magnesium chloride by applying mass/mole analysis
Procedures:
Magnesium will be reacted with hydrochloric acid. In method #1, the mass of the magnesium will be measured and the volume of only enough hydrochloric acid to completely react with the magnesium will be determined. The aqueous product will be isolated and measured. In method #2, magnesium solid will remain and be removed and massed before isolating the aqueous product. The molar ratio of magnesium to chlorine in the product will be calculated. A description of the procedures should explain the reasoning for each step. Data should be clearly recorded.
A.
Label a clean empty 100 or 150 mL beaker with a grease pencil or permanent marker. Measure and record the mass of the empty 100 or 150 mL beaker.
B.
Measure the mass of the beaker with magnesium. Method #2 requires a larger piece of Mg.
Calculate the mass of magnesium to be reacted.
C.
Add the acid slowly - drop by drop. Record the number of drops of hydrochloric acid used.
Method #1. Add only enough HCl to completely react with the magnesium. The entire piece of
Mg must react.
Method #2. Add only enough magnesium to react with approximately half the magnesium.
D.
Record observations of the reaction and magnesium chloride in solution -MgCl
2
(aq).
E.
Determine and record the number of drops per milliliter of your pipet.
(1 mL is approximately 20 drops.)
F.
Calculate the milliliters of HCl added
G. Dry the solution overnight to isolate the magnesium chloride. Method #2. The leftover magnesium solid must be removed and rinsed over the beaker. After drying the mass of the Mg must be measured.
H.
Measure the mass of magnesium chloride remaining in the beaker.
I.
Record observations of solid magnesium chloride MgCl
2
(s).
J.
How would adding excess HCl affect your results?
Think: What would remain in the beaker after drying.
Calculations:
1.
Calculate the moles of magnesium consumed.
2.
a. Calculate the mass of chlorine in the magnesium chloride produced.
b. Calculate the moles of chlorine in magnesium chloride.

3.
Determine the molar ratio of magnesium to chlorine in magnesium chloride.
Show the ratio reduced to simplest whole number ratio.
4.
The actual ratio of magnesium to chlorine is 1:2. Calculate your percent error.
5.
a. Calculate the percent by mass of magnesium in magnesium chloride.
b. Using your percent by mass, if you had 100 grams of magnesium chloride how many grams of magnesium? How many grams of chloride?
Lab Procedure Questions:
6.
a. Compare the limiting reactants in method #1 and method #2. b. What error may be avoided by using method #2? c. Why was the leftover magnesium rinsed over the beaker? d. Why was the solution dried over night?
Additional Calculations using the equation, mass and moles:
7.
THE REACTION Write the balanced formula equation for the reaction. Include states. Refer to your balanced equation for this reaction. One mole of magnesium will require howm amny moles of hydrochloric acid?
8.
Based on the mass of magnesium you started with, how many moles of magnesium chloride could you expect?
9.
Calculate the number of moles of magnesium chloride actually produced.
10.
Calculate the percent yield.
11.
Based on the mass of magnesium you started with, how many moles of hydrochloric acid were required to completely react with the magnesium?
12.
Calculate the number of moles of hydrochloric acid that were actually added. Use the molarity of the hydrochloric acid and the volume used to determine the number of moles.
Analysis of the Reaction:
13.
NET IONIC EQUATION.
Write the total and net ionic equations for the reaction.
14.
HYDROCHLORIC ACID - A REACTANT is an aqueous solution of a strong acid. How is this solution a strong acid? What is the possible pH range for an acidic solution?
15. MAGNESIUM ATOM TO ION.
Compare and describe the magnesium in the reactant to the magnesium in the product. Include bonding, arrangement and # p and #e-. Was magnesium reduced or oxidized? Did it gain or lose electrons? Write the half reaction.
16.
HYDROGEN IONS TO HYDROGEN ATOMS IN A HYDROGEN GAS MOLECULE.
Describe and compare the hydrogen ion in the reactants to the hydrogen atom in the products.
Include bonding, arrangement and # p and #e-. Was it reduced or oxidized? Did gain or lose electrons? Explain.

9.
MAGNESIUM METAL- A REACTANT Magnesium is a metal. Describe magnesium atoms found in the solid magnesium at the beginning of the reaction. Especially the number of electrons and protons and net charge on these atoms. Identify and describe the bond between the atoms of the solid. Explain how this bond allows the solid conducts electricity.
10.
MAGNESIUM CHLORIDE- AN IONIC COMPOUND- APRODUCT Magnesium chloride is an ionic compound. Describe the ions (Include number of protons and electrons).
Especially the magnesium ion so you can compare it to the magnesium atom in the reactants.
Describe how magnesium chloride as a solid is different from the magnesium chloride in solution. What bonds hold together the solid magnesium chloride? What attractions are involved in the solution of magnesium chloride? Explain when it conducts electricity.
HYDROGEN GASA PRODUCT Hydrogen gas is the other product of the reaction. Describe the hydrogen atoms in the hydrogen molecule. Describe the bond that holds together the hydrogen molecule.
Conclusions:
Briefly discuss the accuracy of your results and explain any deviations.

Lab 5 worksheet name _________________________________period_____ to follow magnesium/hydrochloric acid lab zinc reacts with hydrochloric acid in a single replacement reaction
1. Write the balanced equation for the reaction.
2. What happened to the zinc atom? Describe the zinc metal before the reaction and after. Was it oxidized or reduced?
3. What happened to the hydrogen ion? Describe the hydrogen ion before the reaction and after. Was it oxidized or reduced?
4. What happened to the chlorine ion?
5. Show the complete calculation. How many grams of zinc chloride will be produced if .46 g. of zinc are consumed?
6. How many moles of hydrogen gas will be produced?
7. How many milliters of 4M HCl will need to be added in order to completely react the .46 grams of zinc?

Lab #6 worksheet - Empirical Formula Lab Name ____________________period____
The same lab procedures were used to determine the empirical formula of aluminum chloride. Solid aluminum was reacted with only enough hydrochloric acid to completely react with the solid.
LAB DATA: mass of aluminum reacted 1.37 grams mass of aluminum chloride produced 6.92 grams
Calculations based upon the sample data.
1. Calculate the number of moles of aluminum reacted.
2. In the aluminum chloride produced- how many grams are aluminum and how many grams are chlorine? Explain your reasoning.
3. How many moles of chlorine are present in the aluminum chloride.
4. Show and calculate the mole ratio of aluminum to chlorine. Write the empirical formula.
Lab Procedure Questions:
5. Why was only enough HCl added to completely reacted?
6. How was the aluminum chloride isolated?
Concept Questions based on the sample lab.
7.
ALUMINUM a. Describe the solid aluminum before the reaction. Is it an atom or an ion? ________
How many protons? _____ How many electrons? _____ What charge? _______
Describe the bond that holds together the aluminum. b. Describe the aluminum after the reaction. Atom or an ion? ______________
How many protons? _____ How many electrons? ____What charge? _______
Describe the aluminum now. What kind of compound? Where is it?

c. Was the aluminum oxidized or reduced? ______________ Did it lose or gain electrons? ____
Write the half reaction. _______________________ d. Explain why the rate of the reaction increases when powdered aluminum is used instead of aluminum foil.

8. HYDROGEN a. Describe the hydrogen before the reaction. Atom or an ion? ______________
How many protons? _____ How many electrons? _____ What charge? _______
Describe where the hydrogen is at the start. b. Describe the hydrogen after the reaction. Atom or ion? ______________
Number of protons _____ Number of electrons _____ What charge? _______
Describe the hydrogen now. Describe the bond that holds it together. Where is it? c. Was the hydrogen oxidized or reduced? ______________ Did it lose or gain electrons? ____
Write the equation. _________________________
9. CHLORINE a. Describe the chlorine before the reaction. Is it an atom or an ion? ______________
How many protons? _____ How many electrons? _____ What charge? _______
Describe where the chlorine is at the start. b. What happen to the chlorine in the reaction?
10. ALUMINUM CHLORIDE (AlCl
3
) a. What kind of compound? b. Describe it dissolved in solution. c. Describe in the solid form after the water has been removed?
BALANCED EQUATION AND MASS ANALYSIS .
11. Write the balanced chemical equation - aluminum reacts with hydrochloric acid.
12. According to the balanced equation, how moles of aluminum react with how many moles of hydrochloric acid?
13. If you started with 7.84 grams of aluminum, how many moles of hydrochloric acid are required to completely react with the aluminum? Show the dimensional analysis.

14. If 3.92 grams of aluminum react, how many moles of aluminum chloride will be produced?

Empirical Formula Worksheet
The empirical formula name ______________________________per.___
is the smallest whole number ratio of the number of atoms of each element in the substance. For instance, CH
2
is the empirical formula for the series of compounds: C
2
H
4
, C
3
H
6
, C
4
H
8
and so on. The molecular formula is the actual ratio for the molecule.
Solve the following problems to determine the empirical formula. Show the complete factors to calculate number of moles, then the molar ratios and finally the empirical formula.
1.
Calcium bromide is a compound used in photography and the manufacture of fire extinguishing materials.
Analysis of a 20.00 gram sample of calcium bromide yields 4.02 grams of calcium and 15.95 grams of bromine. Calculate the number of moles of both calcium and bromide, then determine the mole ratio to write an empirical formula for calcium bromide.
2.
If 8.87 grams of phosphorus react with 11.43 grams of oxygen, what is the empirical formula of the compound formed?
3.
A small coil of magnesium ribbon is placed in a crucible. The crucible and cover have a mass of 21.35 grams. Measuring the mass, including the magnesium, gives 21.63 grams. The crucible, cover and contents are heated and then cooled and treated with water. A second heating and measuring of the final product
(magnesium combined with oxygen) gives a mass of 21.82 grams. Determine the empirical formula of this oxide of magnesium.
4.
CaSO
4
hydrate was heated to drive off the water of hydration. The mass of the hydrate was 1.26 grams before heating and after heating 0.99 grams of the anhydrous form remained. Find the formula of the hydrate.

5.
NiSO
4
hydrate was heated to drive off the water of hydration. The mass of the hydrate was 1.04 grams before heating and after heating 0.61 grams of the anhydrous form remained. Find the formula of the hydrate.
Use percent composition to determine empirical formulas. Calculate the empirical formula of the compounds with the following percentages.
6. K 40.2%, Cr 26.9% and O 32.9%
7. Mg 21.8%, P 27.9% and O 50.3%
8. Sr 65.7%, Si 10.4% and O 23.9%
9. Three iron oxides. Determine the empirical formula and name the iron oxide.
Fe 77.7% and O 22.3%
Fe 72.4% and O 27.6%
Fe 70.0% and O 30.0%