empirical formula and molecular formulas 1

7/28/2019 Empirical Formula and Molecular Formulas 1

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Empirical Formula

And MolecularFormulas


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Empirical Formula

The smallest whole-number mole ratio forthe elements for a compound.

The data used to determine the chemicalformula for a cmpd may be in the form of

percent composition or it may be theactual masses of the elements in a givenmass of the cmpd.


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Empirical Formulas

If the percent composition is given, youcan assume that the total mass of the

compound is 100 grams The percent by mass of each element is

equal to the mass of that element ingrams.

Example:. The percent composition foroxygen in a sulfur and oxygen compoundis 59.95%. So the percent for sulfur is

40%. What is the empirical formula?


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40 g S 32.07 g S

1 mol S

= 1.249mol S

59.95 g O

16.00 g O

1 mol O = 3.747 mol O

The mole ratio of S atoms to O atoms is 1.249 to 3.747

Can you use these numbers as subscripts?


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Empirical FormulasHow can the mole ratio be converted towhole numbers?

As a starting point recognize that the elementwith the smaller number of moles might havethe smallest subscript possible.

You can make the mole value of sulfur equal to 1 if youdivide both mole values by the value of sulfur (1.25)


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1.249

mol S1.249

= 1 mol S

3.747 mol O

1.25

= 3 mol O

The mole ratio of S atoms to O atoms is1:3 so the empirical formula is SO3


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What the @#$%?

How can I remember? There is a poem to help

1. Percent to Mass

2. Mass to Mole3. Divide by Small

4. Multiply till Whole

Sometimes the mole values are still notwhole numbers so multiply by thesmallest factor that will make them whole

numbers


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Example Problem

Methyl Acetate is a solvent commonlyused in some paints, inks, and adhesives.

Determine the empirical formula for methylacetate which has the following chemicalanalysis: 48.64% C, 8.16% hydrogen, and

43.20% oxygen


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Dont Freak!!!Change percent to mass first then

Mass to mole48. 64 g C

8. 16 g H

43.20 g O

12.01 g C

1.008 g H

16 g O

= 4.050 mol C

= 8.10 mol H

= 2.700 mol O


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Divide each number of moles by

the smallest value in the mole ratio In this case is 2.700

4.050 mol C

8.10 mol H

2.700

2.700

2.700

2.700

= 1.5 mol C

= 3.00 mol H

= 1 mol 0


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What is the ratio, yo?

The simplest ratio is 1.5 mol C: 3 mol H: 1 mol

A decimal cant be in the subscript so we

multiply by the smallest number that will producea ratio of whole numbers

In this case 2 because 2 times 1.5 = 3

2 * 1.5 mol C = 3 mol C

2 * 3 mol H = 6 mol H 2 * 1 mol 0 = 2 mol 0


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The Final Answer

C3H6O2


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Practice Problem!!

Now You Try!!! Dont forget the Poem!!!

Percent to Mass

Mass to mole

Divide by small

Multiply till whole

A blue solid is found to contain 36.84%nitrogen and 63.16% oxygen. What is theempirical formula for the solid?


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Another Practice Problem

Determine the empirical formula for acompound that contains 35.98% aluminum

and 64.02%. Dont forget the poem!!!

Al2S3


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Warm Up

Determine the empirical formula off acompound that is 1.723 g of carbon, .289

g of hydrogen, and .459 g of oxygen.


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An empirical formula represents the

simplest who le number rat io of the

atoms in a compound.

The molecular formulais the t rue or

actual rat io o f the atom s in a

compound .


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Learning Check EF-1

A. What is the empirical formula for C4H8?

1) C2H4 2) CH2 3) CHB. What is the empirical formula for C8H14?

1) C4H7 2) C6H12 3) C8H14

C. What is a molecular formula for CH2O?

1) CH2O 2) C2H4O2 3) C3H6O3


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Solution EF-1

A. What is the empirical formula for C4H8?

2) CH2

B. What is the empirical formula for C8H14?

1) C4H7C. What is a molecular formula for CH2O?

1) CH2O 2) C2H4O2 3) C3H6O3


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Learning Check EF-2

If the molecular formula has 4 atoms of

N, what is the molecular formula if SN is

the empirical formula? Explain.

1) SN

2) SN4

3) S4N4


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Solution EF-2

If the molecular formula has 4 atoms of

N, what is the molecular formula if SN is

the empirical formula? Explain.

3) S4N4

If the actual formula has 4 atoms of N,and S is related 1:1, then there must alsobe 4 atoms of S.


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Empirical and Molecular Formulas

molar mass = a whole number = n

simplest mass

n = 1 molar mass = empirical mass

molecular formula = empirical formula

n = 2 molar mass = 2 x empirical massmolecular formula =

2 x empirical formula

molecular formula = or > empirical formula


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EmpiricalFormula

Empirical

Mass

Molecular

Formula

Molecular

Mass


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Learning Check EF-3

A compound has a formula mass of 176.0

and an empirical formula of C3H4O3. What

is the molecular formula?

1) C3H4O3

2) C6H8O6

3) C9H12O9

empirical formula and molecular formulas 1

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