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Empirical Formula
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Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula?

Slide 12

49.48 C

49.48 C

5.15 H

28.87 N

16.49 O

= 4.1mol

= 5.2mol

= 2.2mol

= 1.0mol

Since they are

close to whole

numbers we will

use this formula

We divide by

lowest (1mol O)

and ratio

doesn’t change

Slide 13

C4.12H5.15N2.1O1

C4.12H5.15N2.1O1

empirical mass = 97g

OR C4H5N2O1

Slide 14

Empirical to molecular

Empirical to molecular

Since the empirical formula is the lowest ratio the actual molecule would weigh more.

By a whole number multiple.

Divide the actual molar mass by the mass of one mole of the empirical formula.

Caffeine has a molar mass of 194 g. what is its molecular formula?

Slide 15

Find x if

Find x if

194 g

97 g

= 2

C4H5N2O1

C8H10N4O2.

2 X

Slide 16

Example

Example

A compound is known to be composed of 71.65 % Cl, 24.27% C and 4.07% H. Its molar mass is known (from gas density) is known to be 98.96 g. What is its molecular formula?

Slide 17

Example

Example

71.65 Cl

24.27C

4.07 H.

= 2.0mol

= 2.0mol

= 4.0mol

Slide 18

Cl2C2H4

Cl2C2H4

Its molar mass is known (from gas density)

is known to be 98.96 g. What is its molecular

formula?

We divide by

lowest (2mol )

Cl1C1H2

would give an empirical wt of 48.5g/mol

Slide 19

Its molar mass is known (from gas density)

Its molar mass is known (from gas density)

is known to be 98.96 g. What is its molecular

formula?

would give an empirical wt of 48.5g/mol

= 2

=

Slide 20

2 X Cl1C1H2

2 X Cl1C1H2

= Cl2C2H4

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