c hapter 2 chemical formulas and composition stoichiometry

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CHAPTER 2 Chemical Formulas and Composition Stoichiometry

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Page 1: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

CHAPTER 2

Chemical Formulas and Composition

Stoichiometry

Page 2: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Chapter Goals

1. Chemical Formulas

2. Ions and Ionic Compounds

3. Names and Formulas of Some Ionic Compounds

4. Atomic Weights

5. The Mole

6. Formula Weights, Molecular Weights, and Moles

7. Percent Composition and Formulas of Compounds

8. Derivation of Formulas from Elemental Composition

9. Determination of Molecular Formulas

10. Some Other Interpretations of Chemical Formulas

11. Purity of Samples

2

Page 3: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

3

Chemical Formulas

• Chemical formula shows the chemical composition of the substance.– ratio of the elements present in the molecule or

compound

• He, Au, Na – monatomic elements• O2, H2, Cl2 – diatomic elements• O3, P4, S8 - more complex elements• H2O, C12H22O11 – compounds

Substance consists of two or more elements

Page 4: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

4

Chemical Formulas

Compound 1 Molecule ContainsHCl 1 H atom & 1 Cl atom

H2O 2 H atoms & 1 O atom

NH3 1 N atom & 3 H atoms

C3H8 3 C atoms & 8 H atoms

Page 5: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

5

Ions and Ionic Compounds

• Ions are atoms or groups of atoms that possess an electric charge.

• Two basic types of ions:– Positive ions or cations

• one or more electrons less than neutral• Na+, Ca2+, Al3+

• NH4+ - polyatomic cation

– Negative ions or anions• one or more electrons more than neutral• F-, O2-, N3-

• SO42-, PO4

3- - polyatomic anions

Page 6: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

6

Ions and Ionic Compounds

• Sodium chloride– table salt is an ionic compound

Page 7: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Systematic Naming

• There are too many compounds to remember the names of them all.

• Compound is made of two or more elements.

• Name should tell us how many and what type of atoms.

Page 8: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Charges on ions

• For most of Group A elements, location on the Periodic Table can tell what kind of ion they form

• Elements in the same group have similar properties.

• Including the charge when they are ions.

Page 9: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

2+

1+

3+ 3- 2- 1-

Charge in groups 1A, 2A and 3A is the group number

Page 10: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Naming Monatomic Ions Naming Monatomic Ions

Page 11: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Name these

Na1+ Sodium ionCa2+ Calcium ionAl3+ Aluminum ion

Li1+ Lithium ion

Page 12: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Write Formulas for these

Potassium ion K1+

Magnesium ion Mg2+

Copper(II) ion Cu2+

Chromium(VI) ion Cr6+

Barium ion Ba2+

Mercury(II) ion Hg2+

Page 13: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Name these

Cl1- Chloride ion N3- Nitride ion Br1- Bromide ion O2- Oxide ion Ga3+ Gallium ion

Page 14: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Write these

Sulfide ion S2-

Iodide ion I1-

Phosphide ion P3-

Strontium ion Sr2+

Page 15: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Binary Ionic Compounds Binary Ionic Compounds

• Binary Compounds – Compounds composed of two different elements

– Binary ionic cmpd, total # of positive and negative charges must equal

Page 16: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Writing Formulas for Binary Ionic Writing Formulas for Binary Ionic Compounds Compounds

• Write the symbols for the ions side by side. ALWAYS write the cation first!

Al3+ O2-

• Cross over the charges by using the absolute value of each ion’s charge as the subscript for the other ion

Al23+

O32-

• Check that the subscripts are in smallest whole number ratio

Al2O3

Page 17: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Naming Binary Ionic CompoundsNaming Binary Ionic Compounds

Al2O3

– 1. Name the cation(don’t change) Aluminum

– 2. Name the anion (drop add-ide) Oxide

Aluminum Oxide

Page 18: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

The Stock System of Nomenclature The Stock System of Nomenclature

• Most transition metals can have more than one type of charge.

• Indicate the charge with Roman numerals in parenthesis.– Fe2+Iron(II) – Fe3+ Iron(III)

• Roman numerals are never used: – For anions – For metals that form

only one ion

Co2+

Cobalt(II) ion

CuCl2

Copper(II) chloride

Page 19: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Naming ions Cont…..

• A few, like silver, zinc and cadmium only form one kind of ion

• Don’t get roman numerals

• Ag+ silver ion

• Zn2+ zinc ion

• Cd2+ cadmium ion

Page 20: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Compounds Containing Polyatomic Compounds Containing Polyatomic Ions Ions

• Naming a series of similar polyatomic ions – NO2

- NO3-

– Nitrite Nitrate– Most common ion is given –ate ending, ion

with one less oxygen -ite

• Naming compounds containing polyatomic ions – Same as for monatomic ions

Page 21: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Naming Binary Molecular Naming Binary Molecular Compounds Compounds

A. Binary Molecular Compounds – 1. Covalently bonded molecules containing only two elements,

both nonmetals B. Naming

– 1. Least electronegative element is named first • Measure of the ability of an atom in chemical compound to

attract e-• Tends to increase across periods decrease or remain the

same down– 2. First element gets a prefix if there is more than 1 atom of that

element – 3. Second element ALWAYS gets a prefix, and an “-ide” ending

• Examples: N2O3 = dinitrogen trioxide

• CO = carbon monoxide, not monocarbon monoxide

Page 22: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

22

Names and Formulas of Some Ionic Compounds

• Table 2-1 gives names of several molecular compounds.– You must know all of the molecular

compounds from Table 2-2.

• Some examples are:– H2SO4 - sulfuric acid– FeBr2 - iron(II) bromide– C2H5OH - ethanol

Page 23: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

23

Names and Formulas of Some Ionic Compounds

• Table 2-2 displays the formulas, charges, and names of some common ions– You must know the names, formulas, and

charges of the common ions in table 2-3.

• Some examples are:– Anions - Cl1-, OH1-, SO4

2-, PO43-

– Cations - Na1+, NH41+, Ca2+, Al3+

Page 24: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Anion Ending Acid Name

-ide hydro-(stem)-ic acid

-ate (stem)-ic acid

-ite (stem)-ous acid

Acid Nomenclature

Page 25: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

C. Johannesson

Acid Nomenclature

h yd ro - p re fix-ic e n d ing

2 e le m e n ts

-a te e nd ingb e com es-ic e nd ing

-ite e nd ingb e com es

-o us e nd ing

n o h yd ro - p re fix

3 e le m e n ts

AC ID Ss ta rt w ith 'H '

Page 26: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

HBr

H2CO3

H2SO3

2 elements, -ide

3 elements, -ate

3 elements, -ite

hydrobromic acid

carbonic acid

sulfurous acid

Acid Nomenclature

Page 27: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

hydrofluoric acid

sulfuric acid

nitrous acid

2 elements

3 elements, -ic

3 elements, -ous

HF

H2SO4

HNO2

Acid Nomenclature

H+ F-

H+ SO42-

H+ NO2-

Page 28: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

28

Names and Formulas of Some Ionic Compounds

You do it!

• What is the formula of nitric acid?

• HNO3

• What is the formula of sulfur trioxide?

• SO3

• What is the name of FeBr3?

• iron(III) bromide

Page 29: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

29

Names and Formulas of Some Ionic Compounds

You do it!

• What is the name of K2SO3?

• potassium sulfite

• What is charge on sulfite ion?

• SO32- is sulfite ion

• What is the formula of ammonium sulfide?

• (NH4)2S

Page 30: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

30

Names and Formulas of Some Ionic Compounds

You do it!

• What is the charge on ammonium ion?

• NH41+

• What is the formula of aluminum sulfate?

• Al2(SO4)3

• What are the charges on both ions?

• Al3+ and SO42-

Page 31: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

31

Page 32: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

32

Atomic Weights

• Weighted average of the masses of the constituent isotopes of an element.– Tells us the atomic masses of

every known element.– Lower number on periodic

table.

Page 33: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

33

The Mole

• A number of atoms, ions, or molecules that is large enough to see and handle.

• A mole = number of things– Just like a dozen = 12 things– One mole = 6.022 x 1023 things

• Avogadro’s number = 6.022 x 1023 – Symbol for Avogadro’s number is NA.

Page 34: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

1 mole of hockey pucks would equal the mass of the moon!

• 1 mole of pennies would cover the Earth 1/4 mile deep!

1 mole of basketballs would fill a bag the size of the earth!

Page 35: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

35

The Mole

Example 2-1: Calculate the mass of a single Mg atom, in grams, to 3 significant figures.

Page 36: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

36

The Mole

Example 2-1: Calculate the mass of a single Mg atom, in grams, to 3 significant figures.

Mg g 104.04atoms Mg mol 1

Mgg24.30

atoms Mg 106.022

atoms Mg mol 1atom Mg 1Mg g ?

23

23

Page 37: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

37

The Mole

Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.

Page 38: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

38

The Mole

Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.

atoms Mg102.48atoms Mgmol 1

atoms Mg106.022

Mgg 24.30

Mg mol 1 Mgg 101.00atoms Mg?

1623

6

Page 39: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

39

The Mole

Example 2-3: How many atoms are contained in 1.67 moles of Mg?

Page 40: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

40

The Mole

Example 2-3. How many atoms are contained in 1.67 moles of Mg?

atoms Mg101.00

Mgmol 1

atoms Mg106.022Mg mol 1.67atoms Mg?

24

23

Page 41: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

41

The Mole

Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg?

You do it!You do it!

Page 42: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

42

The Mole

Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg?

Mg mol 02.3

Mg g 24.30

atoms Mg mol 1 Mg g 4.73Mg mol ?

IT IS IMPERATIVE THAT YOU KNOWHOW TO DO THESE PROBLEMS

Page 43: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

43

Formula Weights, Molecular Weights, and Moles

• How do we calculate the formula weight of a compound?– sum the atomic weight of each atom

The formula weight of propane, C3H8, is:

3 C 3 12.01 amu 36.03 amu

8 H 8 1.01 amu 8.08 amu

Forula Weight 44.11 amu

Page 44: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

44

Formula Weights, Molecular Weights, and Moles

The formula weight of calcium nitrate, Ca(NO3)2, is:

You do it!You do it!

Page 45: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

45

Formula Weights, Molecular Weights, and Moles

amu 164.10 massMolar

amu 96.00 amu 16.006 O6

amu 28.02 amu 14.012 N2

amu 40.08 amu 40.081 Ca1

Page 46: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

46

Formula Weights, Molecular Weights, and Moles

• One Mole of Contains– Cl2 or 70.90g 6.022 x 1023 Cl2 molecules

2(6.022 x 1023 ) Cl atoms

Page 47: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

47

Formula Weights, Molecular Weights, and Moles

Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.

Page 48: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

48

Formula Weights, Molecular Weights, and Moles

Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.

molecules 10 02.1

HC mole 1

molecules HC 106.022

HC g 44.11

HC mole 1

HC g 74.6molecules HC ?

24

83

8323

83

83

8383

Page 49: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

49

Formula Weights, Molecular Weights, and Moles

Example 2-6: What is the mass of 10.0 billion propane molecules?

You do it!You do it!

Page 50: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

50

Formula Weights, Molecular Weights, and Moles

Example 2-6. What is the mass of 10.0 billion propane molecules?

8313

83

83

238310

83

HC of g 1032.7HC mole 1

HC g 11.44

molecules 106.022

HC mole 1molecules 10 1.00 molecules HC g ?

Page 51: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

51

Formula Weights, Molecular Weights, and Moles

Example 2-7: How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O3? The layer of ozone in the stratosphere is very beneficial to life on earth.

You do it!You do it!

Page 52: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

52

Formula Weights, Molecular Weights, and Moles

Example 2-7a. How many moles are contained in 60.0 g of ozone, O3

moles 25.1O g 0.48

mole 1O g 0.60O moles ?

333

Page 53: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

53

Formula Weights, Molecular Weights, and Moles

Example 2-7a: How many moles are contained in 60.0 g of ozone, O3?

Page 54: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

54

Formula Weights, Molecular Weights, and Moles

Example 2-7a. How many moles are contained in 60.0 g of ozone, O3

moles 25.1O g 0.48

mole 1O g 0.60O moles ?

333

Page 55: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

55

Formula Weights, Molecular Weights, and Moles

Example 2-7b: How many molecules are contained in 60.0 g of ozone, O3?

Page 56: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

56

Formula Weights, Molecular Weights, and Moles

Example 2-7b. How many molecules are contained in 60.0 g of ozone, O3

323

23

3

O molecules 107.53

mole 1

molecules 106.022moles 25.1O molecules ?

Page 57: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

57

Formula Weights, Molecular Weights, and Moles

Example 2-7c: How many oxygen atoms are contained in 60.0 g of ozone, O3?

Page 58: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

58

Formula Weights, Molecular Weights, and Moles

Example 2-7c. How many oxygen atoms are contained in 60.0 g of ozone, O3

O atoms 1026.2

molecule O 1

atoms O 3O molecules 107.53atoms O ?

24

33

23

Page 59: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

59

Formula Weights, Molecular Weights, and Moles

Example 2-8: Calculate the number of O atoms in 26.5 g of Li2CO3.

You do it!You do it!

Page 60: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

60

Formula Weights, Molecular Weights, and Moles

Example 2-8. Calculate the number of O atoms in 26.5 g of Li2CO3.

atoms O 106.49

COLi unit formula 1

atoms O 3

COLi mol 1

COLi unitsform.106.022

COLi g 73.8

COLi mol 1COLi g 26.5atoms O ?

23

3232

3223

32

3232

Page 61: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

61

Formula Weights, Molecular Weights, and Moles

• Occasionally, we will use millimoles. – Symbol - mmol– 1000 mmol = 1 mol

• For example: oxalic acid (COOH)2

– 1 mol = 90.04 g – 1 mmol = 0.09004 g or 90.04 mg

Page 62: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

62

Formula Weights, Molecular Weights, and Moles

Example 2-9: Calculate the number of mmol in 0.234 g of oxalic acid, (COOH)2.

You do it!You do it!

Page 63: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

63

Formula Weights, Molecular Weights, and Moles

Example 2-9: Calculate the number of mmol in 0.234 g of oxalic acid, (COOH)2.

22

2

22

(COOH) mmol 2.60(COOH) g 0.09004

(COOH) mmol 1

(COOH) g 234.0(COOH) mmol ?

Page 64: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

64

Page 65: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

65

Percent Composition and Formulas of Compounds

• % composition = mass of an individual element in a compound divided by the total mass of the compound x 100%

Determine the percent composition of C in C3H8.

Page 66: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

66

Percent Composition and Formulas of Compounds

What is the percent composition of H in C3H8?

You do it!You do it!

Page 67: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

67

Percent Composition and Formulas of Compounds

• % composition = mass of an individual element in a compound divided by the total mass of the compound x 100%

Determine the percent composition of C in C3H8.

81.68%

100%g 44.11

g 12.013

100%HC mass

C mass C %

83

Page 68: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

68

Percent Composition and Formulas of Compounds

What is the percent composition of H in C3H8?

81.68%100%18.32%

or

%18.32100%g 44.11

g 1.018

100%HC

H8

100%HC mass

H massH %

83

83

Page 69: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

69

Percent Composition and Formulas of Compounds

Example 2-10: Calculate the percent composition of Fe2(SO4)3 to 3 significant figures.

You do it!

Page 70: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

70

Percent Composition and Formulas of Compounds

Example 2-10: Calculate the percent composition of Fe2(SO4)3 to 3 significant figures.

100% Total

O 48.0% 100%g 399.9

g 16.012 100%

)(SOFe

O12 O %

S 24.1% 100%g 399.9

g 32.13 100%

)(SOFe

S3 S %

Fe 27.9% 100%g 399.9

g 55.82 100%

)(SOFe

Fe2Fe %

342

342

342

Page 71: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

71

Derivation of Formulas from Elemental Composition

• Empirical Formula - smallest whole-number ratio of atoms present in a compound– CH2 is the empirical formula for alkenes– No alkene exists that has 1 C and 2 H’s

• Molecular Formula - actual numbers of atoms of each element present in a molecule of the compound– Ethene – C2H4

– Pentene – C5H10

• We determine the empirical and molecular formulas of a compound from the percent composition of the compound.– percent composition is determined experimentally

Page 72: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Empirical Formula

1. Find mass (or %) of each element.

2. Find moles of each element.

3. Divide moles by the smallest # to find subscripts.

4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.

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73

Derivation of Formulas from Elemental Composition

Example 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula?

Page 74: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

74

Derivation of Formulas from Elemental Composition

Example 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula?Make the simplifying assumption that we have 100.0 g of compound.

O mol 2.531O g 16.00

O mol1O g 40.50 O mol ?

Mn mol 0.6327Mn g 54.94

Mn mol 1Mn g 34.76 Mn mol ?

K mol 0.6327K g 39.10

K mol 1K g 24.74 K mol ?

Page 75: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

75

Derivation of Formulas from Elemental Composition

Example 2-11

4KMnO is formula chemical thethus

O 40.6327

2.531Ofor

Mn 10.6327

0.6327Mnfor K 1

0.6327

0.6327Kfor

rationumber wholesmallest obtain

O mol 2.531O g 16.00

O mol1O g 40.50 O mol ?

Mn mol 0.6327Mn g 54.94

Mn mol 1Mn g 34.76 Mn mol ?

K mol 0.6327K g 39.10

K mol 1K g 24.74 K mol ?

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76

Derivation of Formulas from Elemental Composition

Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is the empirical formula for this compound?

You do it!You do it!

Page 77: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

77

Derivation of Formulas from Elemental Composition

Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is the empirical formula for this compound?

43OCo

:is formula scompound' theThus

O 43O 1.333 Co 33Co 1

number wholetofraction turn to 3 by both multipy

O1.3330.1110

0.1480O for Co 1

0.1110

0.1110Co for

ratio number wholesmallest find

O mol 0.1480O g 16.00

O mol1O g 2.368O mol ?

Co mol 0.1110Cog 58.93

Co mol 1Co g 6.541Co mol ?

Page 78: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Molecular FormulaMolecular Formula

1. Find the empirical formula.2. Find the empirical formula mass.3. Divide the molecular mass by the

empirical mass.4. Multiply each subscript by the

answer from step 3.

nmass EF

mass MF nEF

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79

Determination of Molecular Formulas

Example 2-13: A compound is found to contain 85.63% C and 14.37% H by mass. In another experiment its molar mass is found to be 56.1 g/mol. What is its molecular formula?

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80

Determination of Molecular Formulas

Example 2-13: A compound is found to contain 85.63% C and 14.37% H by mass. In another experiment its molar mass is found to be 56.1 g/mol. What is its molecular formula?

84HC

:is formula theThus

H mol 8H g 1.01

H mol 1H of g 8.10

C mol 4C g 12.0

C mol 1C of g 48.0

moles tomassesconvert

H of g 8.100.1437g 56.1

C of g 48.00.8563g 56.1

H is 14.37% and C is 85.63%

g 56.1 contains mol 1

Page 81: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Law of Multiple Proportions

• It is possible for two elements, A and B, to combine to form more than one compound.

• The ratios of the masses of element B that combine with a given mass of element A in each compound can be expressed by small whole numbers.

81

Page 82: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Law of Multiple Proportions

Example 2-14: Show that the compounds NO2 and N2O5 obey the law of multiple proportions.

82

Page 83: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Law of Multiple Proportions

Example 2-14: Show that the compounds NO2 and N2O5 obey the law of multiple proportions.

83

2

2 5

For NO

22

1

For N O

52.5

2

O atoms

N atoms

O atoms

N atoms

Page 84: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Some Other Interpretations of Chemical Formulas

Example 2-15: What mass of phosphorous is contained in 45.3 grams of (NH4)3PO4?

84

Page 85: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Some Other Interpretations of Chemical Formulas

Example 2-15: What mass of phosphorous is contained in 45.3 grams of (NH4)3PO4?

85

4 3 44 3 4

4 3 4

4 3 4

1 ( )? 45.3 ( )

149.09 ( )

1 30.9738

1 ( ) 1

9.41

mole NH POgrams P g NH PO

g NH PO

mole P g P

mole NH PO mole P

g P

Page 86: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

86

Some Other Interpretations of Chemical Formulas

Example 2-16: What mass of ammonium phosphate, (NH4)3PO4, would contain 15.0 g of N?

Page 87: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

87

Some Other Interpretations of Chemical Formulas

Example 2-16: What mass of ammonium phosphate, (NH4)3PO4, would contain 15.0 g of N?

434434

434434

434434

434

PO)(NH g 53.2PO)(NH mol 1

PO)(NH g 149.0PO)(NH mol 0.357

PO)(NH mol 0.357N mol 3

PO)(NH mol 1N mol 1.07

N mol 1.07N g 14.0

N mol 1N of g 15.0N mol ?

g/mol 149.0PO)(NH of massmolar

Page 88: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

88

Purity of Samples

• The percent purity of a sample of a substance is always represented as

mass of pure substance% purity = 100%

mass of sample

- mass of sample includes impurities

Page 89: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

89

Purity of Samples

Example 2-18: A bottle of sodium phosphate, Na3PO4, is 98.3% pure Na3PO4. What are the masses of Na3PO4 and impurities in 250.0 g of this sample of Na3PO4?

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90

Purity of Samples

Example 2-18: A bottle of sodium phosphate, Na3PO4, is 98.3% pure Na3PO4. What are the masses of Na3PO4 and impurities in 250.0 g of this sample of Na3PO4?

impurities g 4 =

PONa g 246 - sample g 250.0 = impurities g

PONa g 246=

sample g 100.0

PONa g 98.3sample g PONa g

sample g 100.0

PONa g 98.3factor unit

43

43

43

43

43

?

0.250?

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91

Synthesis Problem

In 1986, Bednorz and Muller succeeded in making the first of a series of chemical compounds that were superconducting at relatively high temperatures. This first compound was La2CuO4 which superconducts at 35K. In their initial experiments, Bednorz and Muller made only a few mg of this material. How many La atoms are present in 3.56 mg of La2CuO4?

Page 92: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

92

Synthesis Problem

How many La atoms are present in 3.56 mg of La2CuO4?

Page 93: C HAPTER 2 Chemical Formulas and Composition Stoichiometry

93

Synthesis Problem

How many La atoms are present in 3.56 mg of La2CuO4?

atoms La 1006.1CuOLa molecule

atoms La 2

CuOLa mol 1

CuOLa molecules 10022.6)CuOLa mol 1078.8(

CuOLa mol 1078.8CuOLa g 405.3

CuOLa mol 1

mg 1000

g 1CuOLa mg 3.56

g/mol 405.3=CuOLa of massmolar

19

42

42

4223

426

426

42

42

42

42

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Group Activity

• Within a year after Bednorz and Muller’s initial discovery of high temperature superconductors, Wu and Chu had discovered a new compound, YBa2Cu3O7, that began to superconduct at 100 K. If we wished to make 1.00 pound of YBa2Cu3O7, how many grams of yttrium must we buy?