Chemical Formulas & Chemical Compounds

Chapter 7

Chemical formulas• Common names give no info about

chemical composition.

• Chemists use systematic methods for naming compounds & writing formulas.

A chemical formula indicates the number of atoms of each kind in a

chemical compound.

C8H18

indicates there are indicates there are

8 carbons 18 hydrogen

Monatomic Ions

• Ions formed from a single ion

• Formed by gaining or losing electrons

Cont’d…• Monatomic cations

are identified by the elements name.

• Monatomic anions are named by changing the element’s ending to

–ide.

CationsMg

MagnesiumMg+2

Magnesium cation

AnionsO

OxygenO-2

Oxide

Binary compounds• Compounds composed of two different

elements

• Total number of positive & negative charges must be equal

• The positive & negative charges can be crossed over

Rules for binary compounds1. Write the symbols for the ions side by

side, using the cation first.

2. Cross over the charges.

3. Write the formula without the charges.

ProblemWrite the formula for the binary

ionic compounds formed between the following

elements:

a) magnesium & iodineb) magnesium & sulfur

c) magnesium & nitrogen

Nomenclature• Nomenclature refers to the naming system.• To name binary ionic compounds, combine

names of positive & negative ions.

Al2O3 Aluminum Oxidename of name of

cation anion changing

ending to -ide

Stock system• With some elements (mainly the transition

elements), such as iron, 2 or more cations with different charges are formed

• Uses a Roman numeral to indicate an ion’s charge• Numeral is enclosed in parentheses & placed

immediately after the metal name

CuCl2Copper (II) chloride

name of cation name of+ Roman anion withnumeral for -ide forcharge ending

Oxyanions• Oxyanions are polyatomic ions

containing oxygen.• The most common oxyanions end in –

ate.

NO3- nitrate

NO2- nitrite

Cont’d…• Sometimes more than 2 oxyanions are

formed:

ClO4- perchlorate

ClO3- chlorate

ClO2- chlorite

ClO- hypochlorite

Prefix system• If only one atom of first element in formula,

then no prefix on first element• If more than one atom of the first element,

the first name begins with prefix• Second name begins with a prefix, followed

by the root name, & ending in –ide

P4O10

tetraphosphorus decaoxide

ProblemGive the name for As2O5.

Write the formula for oxygen difluoride.

Acids & Salts• Binary acid: consists of hydrogen & halogen

• Oxyacid: contains hydrogen, oxygen, & a nonmetal

• Salt: ionic compound composed of a cation & the anion from an acid

• Table 7-5 on Page 214: Common acids!

Chemical formulas can be used to calculate the formula mass,

molar mass, & percentage composition.

(1) Formula mass (molar mass)• The sum of the average atomic masses

of all the atoms represented in its formula

PROBLEMFind the formula mass of potassium

chlorate.

(2) Percentage composition• The percentage by mass of each

element in a compound% composition= mass of element x 100 molar mass

PROBLEMFind the percentage composition of copper

(II) sulfide.

Empirical formula• By using the percentage composition, a

substance’s empirical formula can be determined

• Shows the smallest whole number ratio of the different atoms in a compound

Rules1. Change percentage composition to grams.

2. Divide grams of each element by its atomic weight.

3. Change to lowest whole number ratio by dividing each by the smallest number.

ProblemQuantitative analysis shows that

a compound contains 32.38% Na, 22.65% S, & 44.99% O. Find

the empirical formula of this compound.

Molecular formula• Any multiple of an empirical formula

• An empirical formula may or may not be a correct molecular formula

• To determine the molecular formula, you must know the compounds molar mass

ProblemThe empirical formula of a

compound of phosphorus & oxygen was found to be P2O5.

Experimentation shows that the molar mass of this compound is

283.89 g/mol. What is the compound’s molecular formula?