Chapter 7 Naming, Empirical Formula, Molecular Formula and Percent Composition.

Chapter 7

Naming, Empirical Formula, Molecular Formula and Percent

Composition

7.1 Chemical Names and Formulas

C8H18

8 Carbon atoms 18 Hydrogen Atoms

Al2(SO4)3

2 Aluminum atoms 4 Oxygen atoms 3 of everything in ( ) making 3 Sulfur atoms and 12 Oxygen atoms total

Chemical Names & Formulas Oxidation Numbers Using Chemical FormulasDetermining Chemical Formulas Questions

Monatomic Ions:: ions formed from a single atom ::

NAMING :: Element Root + -ide

F F-

Flourine Flouride anion N N3-

Nitrogen Nitride anion

O2- = ?


Binary Ionic CompoundsNomenclature :: naming system

Naming ::

Al2O3

Name of Cation Name of Anion

(with –ide ending)

Aluminum Oxide

Writing the Formula


Nomenclature :: The Stock System

Some elements [such as transition metals] form two or more cations with different charges. The stock system is used to distinguish between the different ions of an element.

Fe2+ Iron (II) Fe3+ Iron (III)

CuCl2 Cu2+ Cl- Name of Cation + Roman Numeral Name of Anion

indicating charge

Copper (II) Chloride


Polyatomic Ions & Oxyanionsoxyanions :: polyatomic ions that contain oxygen

1 less O than ___ite has 1 less O than other oxyanion has 1 more O than other oxyanion 1 more O than ___ate

ClO- ClO2 ClO3 ClO4

Hypochlorite Chlorite Chlorate Perchlorate

Common Polyatomic Ions:

NO3- SO4

2- CO32- PO4

3- OH- NH4+

Nitrate Sulfate Carbonate Phosphate Hydroxide Ammonium


Binary Molecular Compounds

NAMING RULES1. First element does not take a

prefix if it only contributes 1 atom to the compound

2. Second element takes prefix, drop ending, add –ide

3. The o or a at the end of prefix is dropped if the element begins with a vowel


P4O10

Prefix [if needed] + Prefix [determined by # Name of element

atoms] + root name + -ide

Tetraphosphorus decoxide

PREFIXES

# atoms Prefix # atoms Prefix

1 mono- 6 hexa-

2 di- 7 hepta-

3 tri- 8 octa-

4 tetra- 9 nona-

5 penta- 10 deca-

Naming Acids Have O ?

Yes No Poly atomic ion root + suffix Acid Hydro________ic Acid [Hydro + root + ic Acid]

HNO3 Nitric Acid Ex: HCl Hydrochloric acid

Nitrate Nitric Acid HF Hydrofluoric acid HNO2 Nitrous Acid

HClO4 Perchloric Acid

HClO2 Hypochlorous Acid

HClO3 Chloric Acid


ACID SUFFIXES

Polyatomic Ion suffix Acid suffix

- ate [nitrate] -ic [nitric]

-ite [nitrite] -ous [nitrous]

7.2 Oxidation Numbers

Oxidation number :: numbers assigned to compound or ion to indicate the general distribution of electrons among the bonded atoms in a molecular compound/ion

Oxidation numbers are helpful in naming compounds, writing formulas, and balancing chemical equations [redox!!]


Rules for Assigning Oxidation Numbers

1. Atoms in pure element have oxidation number zero (0).2. Elements in a binary molecular compound are given oxidations numbers equal to

the charges they would have as ions3. Flourine always has an oxidation number of -1.4. Oxygen has an oxidation number of -2 in every compound except for peroxides (like

H2O2), where it is -1, and with halogens, has oxidation number +2 (OF2)

5. Hydrogen has oxidation number +1 in compounds with elements more electronegative than itself; it has oxidation number -1 in metal compounds

6. Algebraic sum of oxidation numbers of all atoms in compound add up to charge of compound

7. Monatomic ions have oxidation numbers equal to their charges.8. Rules 1-7 apply to covalently bonded atoms, but oxidation numbers can also be

assigned to atoms in ionic compounds.


Assigning Oxidation Numbers

UF6 We know that fluorine has oxidation # -1. There are 6 F atoms, so -1 x 6 = -6.

Also, the sum of oxidation numbers must equal the charge of the compound, or O, in this case. U + (-6) = 0. Therefore, the Oxidation number of U = +6.

Oxidation Number: U: +6 F: -1

ClO3- Oxygen has oxidation # of -2. Total of oxidation numbers must add up to -1.

Therefore, Cl has oxidation number of +5. Oxidation Number: Cl: +5 O: -2

Oxidation number S in H2SO4 ?? Answer: +6


7.3 Using Chemical Formulas


Formula Mass :: sum of the average atomic masses of all the atoms represented in its formula

FOR EXAMPLE :The formula mass for H2O is:

Average atomic mass for H : 1.01 amuAverage atomic mass of O : 16.00 amu

2 H atoms x 1.01 amu = 2.02 amu1 O atom x 16.00 amu = 16.00 amu

formula mass of H2O = 18.02 amu

© 2009, Prentice-Hall, Inc.

Formula Weights


Formula Weight (FW)• A formula weight is the sum of the atomic

weights for the atoms in a chemical formula.

• So, the formula weight of calcium chloride, CaCl2, would be

Ca: 1(40.1 amu) + Cl: 2(35.5 amu)

111.1 amu

• Formula weights are generally reported for ionic compounds.


Molecular Weight (MW)

• A molecular weight is the sum of the atomic weights of the atoms in a molecule.

• For the molecule ethane, C2H6, the molecular weight would be

C: 2(12.0 amu)

30.0 amu+ H: 6(1.0 amu)


Percent Composition

One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation:

% element =(number of atoms)(atomic weight)

(FW of the compound)x 100


Percent Composition

So the percentage of carbon in ethane is…

%C =(2)(12.0 amu)

(30.0 amu)

24.0 amu

30.0 amu= x 100

= 80.0%


Chemical FormulasThe subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.


Types of Formulas

• Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound.

• Molecular formulas give the exact number of atoms of each element in a compound.


Finding Empirical Formulas


Calculating Empirical Formulas

One can calculate the empirical formula from the percent composition.



The compound para-aminobenzoic acid (you may have seen it listed as PABA on your bottle of sunscreen) is composed of carbon (61.31%), hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the empirical formula of PABA.



Assuming 100.00 g of para-aminobenzoic acid,

C: 61.31 g x = 5.105 mol C

H: 5.14 g x = 5.09 mol H

N: 10.21 g x = 0.7288 mol N

O: 23.33 g x = 1.456 mol O

1 mol12.01 g

1 mol14.01 g

1 mol1.01 g

1 mol16.00 g


Calculating Empirical FormulasCalculate the mole ratio by dividing by the smallest number of moles:

C: = 7.005 7

H: = 6.984 7

N: = 1.000

O: = 2.001 2

5.105 mol0.7288 mol

5.09 mol0.7288 mol

0.7288 mol0.7288 mol

1.458 mol0.7288 mol



These are the subscripts for the empirical formula:

C7H7NO2

Molecular FormulaActual ratio of atoms in a compound. Ex. H2O, C6H12O6

To determine the molecular formula, divide the molar mass by empirical formula mass. This will give the number of empirical formula units (n) in actual molecule.

n= Molar Mass/ Empirical Formula Mass

Ex. Determine the empirical and molecular formula of each of the following: 1.Ethylene glycol, the substance used as antifreeze has 38.70 % C, 9.70 % H and 51.60 % O , mm= 62.10 g2.Caffeine, a stimulant in coffee has the following percent composition:49.50 % C, 5.15% H, 28.90 % N and 16.50 % O , molar mass= 195.00g

Chapter 7 Naming, Empirical Formula, Molecular Formula and Percent Composition.

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Transcript of Chapter 7 Naming, Empirical Formula, Molecular Formula and Percent Composition.