Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an Oxide

Introduction Compounds are pure substances

– a combination two or more elements that form a new compound

Chemical Formula– a combination of symbols of the various elements that make

up the compound

Formula unit– the smallest collection of atoms that provides information on

a compound1. the identity of the atoms2. the relative number of each type of atom

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an Oxide

Law of Definite or Constant Composition

• A pure compound contains 2 or more atoms in definite proportions

• No matter what its source, a specific compound is composed of the same elements in the same mass fraction or ratio

• Every chemical compound has a formula that describes the type and number of different atoms in the compound

• Analysis gives you the composition of AB2C

– mass percentage

• mass % A = (mass of A in the whole amount/mass of the whole amount) x 100%

– molar composition or mole percent

• mole % of A = (moles of A/total number of moles) x 100%

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an Oxide

Question

• What is a mole and what is a molar mass?

• What are the molar masses of magnesium and atomic oxygen?

Answer

• A mole is the quantity of a substance that contains as many molecules or formula units as the number of atoms in exactly 12 g of carbon-12

• A molar mass is the mass of one mole

• The molar masses of magnesium and oxygen are 24.305 g and 15.9994 g, respectively

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an Oxide

MoleChemical counting unit

1 atom Al = 26.98 amu 1 mol Al = 26.98 g1 atom O = 16.00 amu 1 mol O = 16.00 g1 molecule O2 = 32 amu 1 mol O2 = 32.00 g

1 atom Na = 22.99 amu 1 mol Na = 22.99 g1 atom Cl = 35.45 amu 1 mol Cl = 35.45 g1 molecule Cl2 = 70.90 amu 1 mol Cl2 = 70.90 g

1 formula unit NaCl = 58.45 amu 1 mol NaCl = 58.45 g

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an Oxide

Purpose

• Observe the reaction of magnesium with oxygen

• Determine the empirical formula of the product, magnesium oxide

Background

• Oxygen (O2) is very reactive when heated

• Many elements react with oxygen, forming an “oxide”

• Nitrogen (N2) is very unreactive, even at high temperature

• Only very active metals react with N2, forming a “nitride”

• Water (H2O) and nitrides react to form “hydroxides” (compounds of a metal and a hydroxide, OH) and ammonia, NH3.

• Heating the hydroxide converts it to an oxide and water vapor

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an Oxide

Background• Final product is magnesium oxide

1. Primary reaction

Reaction between Mg and O2 MgO

2. Secondary reaction

- Reaction between Mg and N2 Mg3N2

- Mg3N2 and water (H2O) form Mg(OH)2

- Heating Mg(OH)2 results in formation of MgO and H2O

3. All Mg is now converted to MgO

Mass of O2 that reacted with Mg can be determined from the original mass of Mg and the mass of the final product, MgO

4. Laws of conservation of mass and number of moles are used to calculate this amount

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an OxideExperimental ProcedureExperimental Procedure

Prepare• Wash and dry a crucible with lid

• Put the crucible on a clay triangle over a Bunsen burner and heat for about 3 min at high heat

• Cool for 5-10 min (why?) and weigh the crucible with lid

• Repeat heating, cooling and weighing until 2 consecutive weights are within 0.001 g of each other

• Record in your lab notebook in 4 decimals

• Weigh out about 0.2 g Mg ribbon (record weight)

• Fold Mg ribbon loosely (not too tight!) and put in crucible

• Weigh crucible, Mg ribbon and lid. Record weight

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an OxideExperimental ProcedureExperimental Procedure

Heating• Put crucible, Mg ribbon and lid on clay triangle. Cover crucible

with lid.• “Brush” bottom for 2-3 min with hot flame• Put burner under crucible and heat for 3 more min in the hottest

part of the flame• Lift lid slightly with tongs to allow air to enter

– Don’t open too far, because Mg will catch fire– Metal should glow bright-red

• Repeat approximately every 3-5 min until no metal is visible anymore

– all is converted to magnesium oxide powder– no glowing is visible anymore

• Allow crucible to cool

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an OxideExperimental ProcedureExperimental Procedure

Analysis• Remove lid, put few drops of H2O in the crucible

– What do you smell? Report!!• Cover the crucible. Brush the crucible again with the flame until the

contents is dry• Heat on a hot flame for 8-10 min (why?)• Allow crucible to cool down to the point where it is close to room

temperature– you feel no heat when you bring your finger within ½ in of the

crucible• Weigh the crucible + content + lid. Record the weight• Heat again for 3 min• Cool crucible and obtain weight; record weight in 4 decimals• Repeat until weight is constant

– 2 consecutive weightings within 0.001 g of each other

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an OxideExperimental ProcedureExperimental Procedure

Results (1) Mass of empty crucible + lid (g, 4 decimals) ______________

____________________________

(2) Average mass of empty crucible + lid (g) ______________

(3) Mass of crucible, Mg, and lid (g) ______________(4) Mass of Mg [= (3) – (2)] (g) ______________

After heating(5) Mass of crucible, lid and oxide (g) ______________

____________________________

(6) Average mass (g) ______________(7) Average mass of empty crucible + lid [=(2)] (g) ______________(8) Mass of oxide (g) ______________

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an OxideExperimental ProcedureExperimental Procedure

Calculations

1. Mass of O = mass of MgO – mass of Mg __________

2. Moles of Mg = mass of Mg/molar mass of Mg __________

3. Moles of O = mass of O/molar mass of O__________

4. Molar ratio of Mg:O = __________

5. Empirical formula for magnesium oxide =

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an OxideExperimental ProcedureExperimental Procedure

Balanced equations1. Mg with molecular oxygen

Mg + O2

2. Mg with molecular nitrogenMg + N2

3. Magnesium nitride with waterMgyNz + H2O Mg(OH)x

4. Heating magnesium hydroxideMg(OH)X + heat Mg-oxide + H2O

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an OxideChemicalsChemicals

Post-Lab Assignment1. Exp 3A: The Empirical Formula of an Oxide

– Fill in the Results sheet (p. 65)

– Show all your calculations. Calculations need to be complete and logically organized and written

• Analysis– Determine the empirical formula for the oxide of magnesium.

– Think carefully about the precision of the masses that you determined on the electronic balance. How many significant figures are justified in your answer?

• Answer questions 1 and 2 on Post-Lab Assignments sheet

Exp 3A: The Empirical Formula of an OxideExp 3A: The Empirical Formula of an OxideChemicalsChemicals

• Due Next Week– Exp. 3A:

Data/results Sheet Post-lab Questions

– Exp. 3B: Hydrates and their Thermal Decomposition• Prelab assignments 1 - 4• Read and understand

– Goal of experiment– Procedure

– Properties of BaCl2, CrCl3, CoCl2, NiSO4, CuSO4 and the hydrates mentioned in the lab text