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2

Chapter Goals

1.Chemical Formulas

2. Ions and Ionic Compounds

3.Names and Formulas of Some Ionic

Compounds4.Atomic Weights

5.The Mole



3

Chapter Goals

6.Formula Weights, Molecular Weights, andMoles

7.Percent Composition and Formulas of Compounds

8.Derivation of Formulas from ElementalComposition

9.Determination of Molecular Formulas

10.Some Other Interpretations of ChemicalFormulas

11.Purity of Samples



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Chemical Formulas

• Chemical formula shows the chemicalcomposition 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



5

Chemical Formulas

Compound 1 Molecule Contains HCl 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



6

Ions and Ionic Compounds

• Ions are atoms or groups of atoms thatpossess 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



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Ions and Ionic Compounds

• Sodium chloride

– table salt is an ionic compound



9

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- , SO42- , PO4

3-

– Cations - Na1+ , NH41+ , Ca2+ , Al3+



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Names and Formulas of Some Ionic Compounds• Formulas of ionic compounds are determined by

the charges of the ions. – Charge on the cations must equal the charge on

the anions.

– The compound must be neutral.

• NaCl sodium chloride (Na1+ & Cl1- )

• KOH potassium hydroxide(K1+ & OH1- )

•CaSO

4calcium sulfate (Ca2+ & SO

4

2- )

• Al(OH)3 aluminum hydroxide (Al3+ & 3 OH1- )



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Names and Formulas of Some Ionic Compounds

You do it!

• What is the formula of nitric acid?

•HNO

3• What is the formula of sulfur trioxide?

• SO3

• What is the name of FeBr 3?• iron(III) bromide



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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



13


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 SO4

2-



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Atomic Weights

• Weighted average of themasses of the constituentisotopes of an element.

– Tells us the atomic massesof every known element.

– Lower number on periodictable.

• How do we know what thevalues of these numbersare?



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The Mole

• A number of atoms, ions, or moleculesthat is large enough to see and

handle.• A mole = number of things

– Just like a dozen = 12 things

–One mole = 6.022 x 10

23

things• Avogadro’s number = 6.022 x 1023

– Symbol for Avogadro’s number is NA.



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The Mole

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



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The Mole

Example 2-2: Calculate the number of atoms inone-millionth of a gram of Mg to 3 significantfigures.



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The Mole

Example 2-3: How many atoms are contained in1.67 moles of Mg?



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The Mole

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

You do it! You do it!



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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 amu8 H 8 1.01 amu 8.08 amu

Forula Weight 44.11 amu

× = × =

× = × =

=



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Formula Weights, Molecular Weights, and MolesThe formula weight of calcium nitrate, Ca(NO3)2,

is:




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

2(6.022 x 1023 ) Cl atoms

– C3H8




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Formula Weights, Molecular Weights, and MolesExample 2-5: Calculate the number of C3H8

molecules in 74.6 g of propane.



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Formula Weights, Molecular Weights, and MolesExample 2-6: What is the mass of 10.0 billionpropane molecules?




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Formula Weights, Molecular Weights, and MolesExample 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.




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Formula Weights, Molecular Weights, and MolesExample 2-7a: How many moles are contained in60.0 g of ozone, O3?



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Formula Weights, Molecular Weights, and MolesExample 2-7b: How many molecules arecontained in 60.0 g of ozone, O3?



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Formula Weights, Molecular Weights, and MolesExample 2-7c: How many oxygen atoms arecontained in 60.0 g of ozone, O3?



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Formula Weights, Molecular Weights, and MolesExample 2-8: Calculate the number of O atoms in26.5 g of Li2CO3.




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• 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



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Formula Weights, Molecular Weights, and MolesExample 2-9: Calculate the number of mmol in0.234 g of oxalic acid, (COOH)2.




32

Percent Composition andFormulas of Compounds

• % composition = mass of an individualelement in a compound divided by thetotal mass of the compound x 100%

Determine the percent composition of C in C3H8.

•

•



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Percent Composition andFormulas of CompoundsWhat is the percent composition of H in C3H8?




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Percent Composition andFormulas of CompoundsExample 2-10: Calculate the percent compositionof Fe2(SO4)3to 3 significant figures.

You do it!



35

Derivation of Formulas fromElemental 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 eachelement 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 thecompound. – percent composition is determined experimentally



36

Derivation of Formulas fromElemental Composition• We determine the empirical and molecular formulas of a

compound from the percent composition of thecompound.

– percent composition is determined experimentally



37

Derivation of Formulas fromElemental CompositionExample 2-11: A compound contains 24.74% K,34.76% Mn, and 40.50% O by mass. What is itsempirical formula?



38

Derivation of Formulas fromElemental CompositionExample 2-12: A sample of a compound contains6.541g of Co and 2.368g of O. What is theempirical formula for this compound?




39

Determination of Molecular Formulas

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



40

Law of Multiple Proportions

• It is possible for two elements, A and B, tocombine to form more than onecompound.

• The ratios of the masses of element B thatcombine with a given mass of element Ain each compound can be expressed by

small whole numbers.



41

Law of Multiple Proportions

Example 2-14: Show that the compounds NO2

and N2O5 obey the law of multiple proportions.

S Oth I t t ti f



42

Some Other Interpretations of Chemical FormulasExample 2-15: What mass of phosphorous iscontained in 45.3 grams of (NH4)3PO4?

S Oth I t t ti f



43

Some Other Interpretations of Chemical Formulas

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



44

Purity of Samples

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

•

mass of pure substance% purity = 100%mass of sample

- mass of sample includes impurities

×



46

Synthesis Problem

In 1986, Bednorz and Muller succeeded in makingthe first of a series of chemical compounds thatwere superconducting at relatively hightemperatures. This first compound wasLa2CuO4which superconducts at 35K. In their

initial experiments, Bednorz and Muller made onlya few mg of this material. How many La atoms arepresent in 3.56 mg of La

2CuO

4?



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Synthesis Problem

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



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Group Activity

• Within a year after Bednorz and Muller’sinitial discovery of high temperature

superconductors, Wu and Chu haddiscovered a new compound,YBa2Cu3O7, that began to superconduct

at 100 K. If we wished to make 1.00

pound of YBa2Cu3O7, how many gramsof yttrium must we buy?



49

2Chemical Formulas

and CompositionStoichiometry

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