Exp 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

a compound collection of atoms that provides information on 1. the

identity

of the atoms 2. the

relative number

of each type of atom

Exp 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 AB 2 C – 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 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 Oxide Mole Chemical counting unit 1 atom Al = 26.98 amu 1 atom O = 16.00 amu 1 molecule O 2 = 32 amu 1 atom Na = 22.99 amu 1 atom Cl = 35.45 amu 1 molecule Cl 2 = 70.90 amu 1 formula unit NaCl = 58.45 amu 1 mol Al = 26.98 g 1 mol O = 16.00 g 1 mol O 2 = 32.00 g 1 mol Na = 22.99 g 1 mol Cl = 35.45 g 1 mol Cl 2 = 70.90 g 1 mol NaCl = 58.45 g

Exp 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 (O 2 ) is

very reactive

when heated Many elements react with oxygen, forming an “

oxide

” Nitrogen (N 2 ) is

very unreactive

, even at high temperature Only very active metals react with N 2 , forming a “

nitride

” Water (H 2 O) and nitrides react to form “

hydroxides

” (compounds of a metal and a hydroxide, OH) and ammonia, NH 3 .

Heating the hydroxide converts it to an oxide and water vapor

Exp 3A: The Empirical Formula of an Oxide • Background Final product is magnesium oxide 1.

Primary reaction Reaction between Mg and O 2 

MgO

2.

Secondary reaction - Reaction between Mg and N - Mg 3 N 2 2 

Mg 3 N 2

and water (H 2 O) form

Mg(OH) 2

- Heating Mg(OH) 2 results in formation of

MgO

and H 2 O 3.

All Mg is now converted to

MgO

Mass of O 2 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 Oxide Experimental 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 Oxide Experimental 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 Oxide Experimental Procedure • • • • • • • • Analysis Remove lid, put few drops of H 2 O 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 Oxide Experimental Procedure Results (1) Mass of empty crucible + lid (g, 4 decimals) (2) Average mass of empty crucible + lid (g) ______________ ______________ ______________ ______________ (3) (4) Mass of crucible, Mg, and lid (g) Mass of Mg [= (3) – (2)] (g) ______________ ______________ After heating (5) (6) (7) (8) Mass of crucible, lid and

oxide

(g) Average mass (g) Average mass of empty crucible + lid [=(2)] (g) Mass of

oxide

(g) ______________ ______________ ______________ ______________ ______________ ______________

Exp 3A: The Empirical Formula of an Oxide Experimental Procedure Calculations 1.

Mass of O = mass of MgO – mass of Mg 2.

3.

4.

Moles of Mg = mass of Mg/molar mass of Mg Moles of O = mass of O/molar mass of O Molar ratio of Mg:O = __________ __________ __________ __________ 5.

Empirical formula for magnesium oxide =

Exp 3A: The Empirical Formula of an Oxide Experimental Procedure Balanced equations 1.

Mg with molecular oxygen Mg + O 2  2.

Mg with molecular nitrogen Mg + N 2  3.

Magnesium nitride with water Mg y N z + H 2 O  Mg(OH) x 4.

Heating magnesium hydroxide Mg(OH) X + heat  Mg-oxide + H 2 O

Exp 3A: The Empirical Formula of an Oxide Chemicals Post-Lab Assignment 1.

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 Oxide Chemicals

• 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 BaCl 2 , CrCl 3 , CoCl 2 , NiSO 4 , CuSO 4 hydrates mentioned in the lab text and the