This document provides the procedure for determining the empirical formula of a compound through gravimetric analysis. Students will react magnesium metal with oxygen in air to produce a magnesium oxide compound. They will collect and weigh the products, then calculate the mole ratios of magnesium and oxygen to determine the empirical formula. The procedure involves weighing reactants and products, calculating masses of each element present, converting to moles, and determining the simplest whole number ratio to give the empirical formula.

1. October 10, 2023
Week #5: Determining
the Empirical Formula
of a Compound

2. Purpose of the
Experiment
 Establish the empirical formal of
a compound from the gravimetric
determination of the molar ratio
of the reacting masses of the
elements.
Gravimetric determination-
through the measurement of mass.

3. Background Information
 The law of conservation of matter: Matter is neither gained nor lost during a chemical
reaction

4.  In a given compound, the constituent elements are always combined in the same proportions
(Law of definite proportions)
This is true regardless of the source of the compound or its method of preparation
 The formula of a compound can be determined from the combining masses of the reactants
and the mass of the product as long as the compound follows the law of definite proportions.
 Empirical Formula: Simplest whole number ratio of the relative numbers of atoms of each
element in the compound
 Molecular Formula: Actual number of atoms of each element in the compound
Background Information

5. How to calculate empirical mass and molecular mass
Compound
Molecular
Formula
Empirical
Forumla
Forula Weight
Glucose C6H12O6 CH2O 180.156 g/mol
Mol mass of H:1.01 g/mol
Mol mass of C: 12.01 g/mol
Mol mass of O: 16.00 g/mol

6. Finding the
Empirical
Formula
First find the mass of each
element in the sample of a
compound.
Convert the masses to moles of
each element.
Then determine the ratio.

7. Finding the mass of the constituent elements

8. Convert masses to moles
Determine the ratio
mole =
𝑚𝑎𝑠𝑠
𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠

9. Experiment
1. Heat Mg in the air
__Mg (s) + __O2(g)  MgxOy (s)
3Mg (s) + N2(g)  Mg3N2(s)
2. Add water
Mg3N2(s) +6H2O(l)  3Mg(OH)2(s) + 2NH3(g)
3. Heat Mg(OH)2
Mg(OH)2(s) + heat  MgxOy (s) + O2(g)
Calculate the number of moles of Mg reacting with oxygen
Calculate the number of moles of oxygen
Determine the empirical of formula of MgxOy
__Mg (s) + __O2(g)  MgxOy (s)

10. Procedure
 Place a clean, dry crucible with a cover on a clay triangle.
 Position the iron ring so the bottom of the crucible is ~1 cm above the inner blue
cone of the burner flame.
 Heat the crucible and cover for 2 minutes, allow to cool.
 Weigh the crucible and cover to the nearest 0.001 g. Record.
 Reheat crucible and cover, cool, weigh. Record again.
 Add about 0.2 g of Mg ribbon to the crucible packed into the bottom.
 Weigh the crucible, crucible cover, and Mg to the nearest 0.001 g. Record.
 Place the crucible on the clay triangle with the crucible cover tilted slightly open
to allow O2 in the air to react with Mg.
 Heat gently at first, then more briskly.
 If Mg starts to burn and smoke is given off, cover the crucible completely and
remove the flame for a short time.
 When contents no longer glow bright red on heating and smoke is not gently given
off, the reaction is complete.

11. Procedure
 Briskly heat the crucible for several minutes with the cover tilted open, until the
bottom of the crucible glows a dull red.
 Allow the crucible and the contents to cool.
 Carefully push the fluffy product down into the bottom of the crucible with a clean,
glass stirring rod.
 Scrape particles adhering to the rod into the crucible.
 Add 6 to 8 drops of dH2O. Note the odor of the NH3.
 Heat the crucible with the cover in place, gently then more briskly until the
crucible glows dull red. Continue heating for 5 minutes.
 Cool and weigh the crucible and contents. Record.
 Reheat, cool, reweigh until mas is constant to within 0.02 g. Record.
 Repeat the entire procedure 2x more with new samples of Mg.

12. Procedure
https://www.youtube.com/watch?v=1Mi7_O07k_M

13. Calculations
 Calculating mass of Mg (g)
Mass of Mg=(mass of crucible and cover and Mg)-(mass of crucible and cover after heating) (g)
 Calculating mass of product (MgxOy) (g).
Mass of MgxOy=(mass of crucible, cover and product)-(mass of crucible and cover after heating) (g)
 Calculating mass of O reacted
Mass of O=(mass of MgxOy)-(mass of Mg) (g)
 Calculating moles of Mg, mol
 Calculating moles of O, mol
 Calculating ratio/empirical formula

14. MgxOy data 1 2 3
mass (crucible + cover)/ g 28.89378 28.89667 28.89956
mass (crucible + cover reheating)/ g 28.87644 28.88511 28.87644
mass (crucible + cover + Mg)/ g 29.07044 29.07911 29.07244
mass (crucible + cover + product)/ g 29.20687 29.2097 29.20435
mass (crucible + cover + product reheating)/ g 29.19227 29.20386 29.18975
Data

15. Data sheet and calculations
1 2 3
1 Mass of crucible and cover, g
2 Mass of crucible and cover after heating, g
3 Mass of crucible, cover, and Mg, g
4 Mass of crucible and cover after heating, g 2
5 Mass of Mg, g 5=3-4
6 Mass of crucible, cover, and product, g
7 Mass of crucible and cover after heating, g 2
8 Mass of product, g 8=6-7
9 Mass of crucible, cover, and product after reheating to
constant mass, g
10 Mass of crucible and cover after reheating, g 2
11 Mass of product, g 11=9-10
12 Mass of Mg used, g 5
13 Mass of O in the product 13=11-12
14 Number of moles reacting Mg, mol 14=5/mola mass (Mg)
15 Number of moles reacting O, mol 15=13/mola mass (O)
16 Ratio of number of moles of Mg to that of O 16=14/15
17 Empirical formular of product
Molar mass of Mg=24.305 g/mol
Molar mass of O=15.990 g/mol

16. Summary
Finding the Empirical Formula of
compound (AxBy)
Mass of A (g) measured
Mass of product, AxBy (g) measured
Mass of B (g)
Mole of A used
Mole of B used
Ratio of number of moles of A to the number of moles of B
Empirical formula, AxBy

17. Lab report
Experimental and Discussion
Data sheets, pp. 51
Post Lab Question, pp. 49-50
Lab report due next lab experiment (see syllabus)
Next Lab: Properties and reaction of acids and
bases (pp.85-92