It gives the lowest term ratio of atoms (or moles) in a formula.

Ex. C4H10 (butane) --> molecular fomula

Reduce the subscripts to lowest terms to get the empirical formula

C2H5 --> empirical formula

Ex. Consider that we have 10.87 grams of Fe and 4.66 g of O. What is the empirical formula?

First, we must convert the grams to moles.

10.87g x

__1mole__= 0.195 moles of Fe

55.8g

4.66g x

__1 mole__= 0.291 moles of O

16g

Now, we divide the mole amount by

**the smallest mole**which is 0.195

Fe:

__0.195__= 1

0.195

O:

__0.291__= 1.49 = 1.5

0.195

Next, we must get those two numbers to the

**smallest whole number**

Fe 1 x 2 = 2

O 1.5 x 2 = 3

The empirical formula is: Fe2O3

Ex. A compound contains 31.9% of K, 28.9% of Cl, and 39.2% of O. What is the empirical formula?

*assume we have 100g*

31.9 x

__1 mol__= 0.816 moles of K

39.1g

28.9 x

__1 mol__= 0.814 moles of Cl

35.5g

39.2 x

__1 mol__= 2.45 moles of O

16g

lowest mole: 0.814

__0.816__= 1.00

0.814

__0.814__= 1

0.814

__2.45__= 3

0.814

Empirical formula: KClO3

What is the molecular formula?

It is a multiple of the empirical formula and shows the actual numbers of atoms that combine to form a molecule.

To calculate the multiple, we use this formula:

n =

__molar mass of the compound__

molar mass of the empirical formula

Ex. A molecule has an empirical formula of C2H5 and the molar mass is 58 g/mol. What is the molecular formula?

Molar mass is C2H5 = 58 g/mol

Molar mass of the empirical formula (total molar mass of all elements in the compound):

C2 = 12 x 2 = 24

H5 = 1 x 5 = 5

C2H5 = 29 g/mol

__58 g/mol__= 2

29 g/mol

2 x C2H5 = C4H10

Video about the empirical formula:

Video about the molecular formula:

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