Empirical FormulaEmpirical Formula

The simplest formula that The simplest formula that represents the whole represents the whole number ratio between the number ratio between the elements in a compound.elements in a compound.

Reduce the subscripts in aReduce the subscripts in a

formula to their simplestformula to their simplest

whole number ratio.whole number ratio.

Empirical FormulaEmpirical Formula

The molecular formula forThe molecular formula for

hydrogen peroxide is Hhydrogen peroxide is H22OO22,,

its empirical formula is HO.its empirical formula is HO.

Empirical FormulaEmpirical Formula

Subscripts areSubscripts are

usually thought ofusually thought of

as a ratio of atoms. as a ratio of atoms.

Empirical FormulaEmpirical Formula

Subscripts can also beSubscripts can also be

thought of as a ratio ofthought of as a ratio of

moles of atoms. moles of atoms.

Empirical FormulaEmpirical Formula

To determine anTo determine an

empirical formula,empirical formula,

one must determineone must determine

the mole ratio.the mole ratio.

Empirical FormulaEmpirical Formula

A mole is the number of

atoms needed to equal

the atomic mass of

an element.

MolesMoles

The atomic mass of

an element in grams

is equal to one mole

of its atoms.

MolesMoles

One mole of anything

is equal to 6.02 X 1023

MolesMoles

Determine the empirical

formula of a compound that

is composed of 36.5% Na,

25.4% S, and 38.1% O.

Sample ProblemSample ProblemSample ProblemSample Problem

You can count on the

elements being given

in the problem in the

same order they are

found in the formula.

HINT:HINT:HINT:HINT:

Find the number

of moles of each

element present.

Step 1Step 1

Since the amount of each

element is given in

percentage, you must convert

the percentage to a mass...

Step 1Step 1

If 100 grams of the sample are

assumed, the percentages

given are the same as grams.

Step 1Step 1

Step 1Step 1

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Determine the whole

number mole ratio.

Step 2Step 2

Step 2Step 2

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Use the mole ratio for

the subscripts of each

element in the formula.

Step 3Step 3

The empirical formula is

Na2SO3

Step 3Step 3

The formula mass of the sampleThe formula mass of the sample

problem is 126 u. If theproblem is 126 u. If the

molecular mass is known to bemolecular mass is known to be

378 u, what is the molecular378 u, what is the molecular

formula?formula?

Finding a molecular formula Finding a molecular formula from an empirical formulafrom an empirical formula

Divide the molecular mass

by the empirical formula

mass to get a whole

number.

SolutionSolution

378126 = 3

Multiply each subscript

in the empirical formula

by this whole number.

SolutionSolution

You now have the

molecular formula -

Na6S3O9

SolutionSolution

Hydrated Crystals