Exp 7: Empirical FormulasExp 7: Empirical Formulas

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 7: The Empirical Formula of an OxideExp 7: 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

• Interpretation of a Chemical Formula:

– By pieces of matter: atoms, molecules, ions

– By the number of moles: where a mole is a “bunch” of pieces of matter

• 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 7: The Empirical Formula of an OxideExp 7: The Empirical Formula of an OxideQuestion

• 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 mole is a quantity of substance whose mass in grams is numerically equal to its mass in amu

• A molar mass is the mass in grams of one mole of a substance

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

Exp 7: The Empirical Formula of an OxideExp 7: The Empirical Formula of an Oxide

Mole is also considered a Chemical counting unit1 atom Al = 26.98 amu 1 mol Al = 26.98 g

1 atom O = 16.00 amu 1 mol O = 16.00 g

1 molecule O2 = 32 amu 1 mol O2 = 32.00 g

1 atom Na = 22.99 amu 1 mol Na = 22.99 g

1 atom Cl = 35.45 amu 1 mol Cl = 35.45 g

1 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 7: The Empirical Formula of an OxideExp 7: The Empirical Formula of an OxideNOTES for the EXPERIMENT

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 7: The Empirical Formula of an OxideExp 7: 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 7: The Empirical Formula of an OxideExp 7: 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 7: The Empirical Formula of an OxideExp 7: 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 7: The Empirical Formula of an OxideExp 7: The Empirical Formula of an OxideExperimental ProcedureExperimental Procedure

Analysis• When the crucible has cooled 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 7: The Empirical Formula of an OxideExp 7: The Empirical Formula of an OxideChemicalsChemicals

Post-Lab Assignment due next week• Fill in the Report Sheet (p. 115)

– Omit 9, 10, 11

– Show all your calculations on a separate sheet of paper.

• Answer questions 1 through 4 on Laboratory questions following your calculations on the sheet of paper

• Critical Thinking (no need to write):

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

Also due: Pre-lab for next lab