Title: Exp 3A: The Empirical Formula of an Oxide
1Exp 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 compound - the identity of the atoms
- the relative number of each type of atom
2Exp 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
3Exp 3A The Empirical Formula of an Oxide
- Mass percentage and empirical formula
- 0.192 g P reacts with oxygen to form 0.440 g of a
phosphorus oxide - mass of oxygen in the oxide
- (total mass of the oxide) (mass of
phosphor) - (0.440 g 0.192 g) 0.248 g oxygen
- Mass O (0.248 g/0.440 g) x 100 56.4
- Mass P (0.192 g/0.440 g) x 100 43.6
4Exp 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
5Exp 3A The Empirical Formula of an Oxide
- Mole
- Chemical counting unit
- 1 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
6Exp 3A The Empirical Formula of an Oxide
- Mole percentage and empirical formula
- 0.192 g P reacts with oxygen to form 0.440 g of a
phosporus oxide - mass of oxygen in the oxide 0.248 g oxygen
- molar mass of P 30.98 g/mol
- molar mass of O 15.9994 g/mol
- of moles of O 0.248 g/15.9994 g/mol 0.0155
mol 1.55 x 10-2 mol - of moles of P 0.192 g/30.974 g/mol 0.0062
mol 6.2 x 10-3 mol - We have 1.55 x 10-2 mol O and 6.2 x 10-3 mol P
- Ratio O/P 1.55 x 10-2 mol/ 6.2 x 10-3 mol 2.5
- Empirical formula P1O2.5
- You cannot split atoms ? it needs to be a
whole-number ratio - ? P1O2.5 P2O5 (empirical formula)
- Molar percentage of P 2 mol/7 mol x 100 28.6
- Molar percentage of O 5 mol/7 mol x 100 71.4
7Exp 3A The Empirical Formula of an Oxide
- Possible Reaction Equation
- Elemental phosphor P4
- Elemental oxygen O2
- Reaction equation
- P4 O2 ? P2O5
- Needs balancing (Law of Mass Conservation)
- number of atoms in the reactants (left of the
arrow) number of atoms in product(s) (right of
the arrow) - balanced equation
- P4 5 O2 ? 2P2O5
8Exp 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
9Exp 3A The Empirical Formula of an Oxide
- Background
- Final product is magnesium oxide
- Primary reaction
- Reaction between Mg and O2 ? MgO
- 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 - 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 - Laws of conservation of mass and number of moles
are used to calculate this amount
10Exp 3A The Empirical Formula of an
OxideExperimental 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
11Exp 3A The Empirical Formula of an
OxideExperimental 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 - Dont 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
12Exp 3A The Empirical Formula of an
OxideExperimental 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
13Exp 3A The Empirical Formula of an
OxideExperimental 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) ______________
14Exp 3A The Empirical Formula of an
OxideExperimental Procedure
- Calculations
- Mass of O mass of MgO mass of Mg __________
- Moles of Mg mass of Mg/molar mass of
Mg __________ - Moles of O mass of O/molar mass of
O __________ - Molar ratio of MgO __________
- Empirical formula for magnesium oxide
15Exp 3A The Empirical Formula of an
OxideExperimental Procedure
- Balanced equations
- Mg with molecular oxygen
- Mg O2 ?
- Mg with molecular nitrogen
- Mg N2 ?
- Magnesium nitride with water
- MgyNz H2O ?Mg(OH)x
- Heating magnesium hydroxide
- Mg(OH)X heat ? Mg-oxide H2O
16Exp 3A The Empirical Formula of an
OxideChemicals
- Magnesium
- Formula Mg
- Mole mass 24.305
- Melting point 650 oC
- Boiling point 1107 oC
- Density 1.74 g/cm3
- Water Solubility Insoluble/reactive
- Appearance Silvery to grayish-white metal
- Comments Flammable solid
- Magnesium oxide
- Formula MgO
- Mole mass 40.3044
- Melting point 2800
- Boiling point 3600
- Density 3.58 g/cm3
- Water Solubility Insoluble
- Appearance White fume
- Comments Moisture-sensitive
- Oxygen
- Formula O2
- Mole mass 31.9988
- Melting point -218.4 oC
- Boiling point -183.17 oC
- Vapor Density 1.1053 g/cm3
- Water Solubility Slightly soluble
- 0.004 g/100 ml
- Appearance Colorless gas blue liquid
- Comments non-flammable gas
17Exp 3A The Empirical Formula of an
OxideChemicals
- Post-Lab Assignment
- 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
18Exp 3A The Empirical Formula of an
OxideChemicals
- 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