Empirical and Molecular Formula - Isabelle Cheng

Isabelle Cheng

Block 2-2 Chemistry 11

Ms. Chen

Empirical and Molecular Formula

Empirical - smallest whole number ratio of atoms which represent the molecular composition of a species. In other words “species” means elements. It is where a number the numbers can be divided by a certain number. For example, C8H16 it would be reduced down to C2H4. That is the lowest terms in this case.

Here is one example of doing an Empirical Formula question.

Say that you have 54.09% of Ca, 43.1% of O and 2.73% H. What is the empirical formula?

The first step:

Separate them into elements. 

Ca

O

H

The second step:

Now you want to convert grams into moles. This means you have to write the mass on the side now. 

Ca =  54.09

O = 43.1

H = 2.73

The third step:

Now you have to multiply it by one mole and divide it by the mass of the element (species).

Ca = 54.09/40.1 = 1.349

O = 43.16/ 16= 2.699

H = 2.73/1 = 2.73

   

The fourth step:

Now divide both by the smallest molar amount.

Ca = 1.349/1.349 = 1

O = 2.699/1.352 = 2

H = 2.73/1.352 = 2

The fifth step:

Now put them together:

Ca(OH)2 which is Calcium Hydroxide!

Now moving onto molecular formula!

Molecular Formula - it is a multiple of many empirical formula and has the real number of atoms that combine to form a molecule!

The formula for this is:

Let N = the WHOLE NUMBER MULTIPLE OF THE EMPIRICAL MASS:

multiple N =       Molar mass

Empirical mass

One example of the Molecular Formula is this:

A hydrocarbon is 84.25% carbon and 15.75% hydrogen and has a molecular weight of 114. What is the molecular formula?

C = 84.25/12 = 7.021

H = 15.75/1 = 15.75

Then you go:

7.021/7.021 = 1

15.78/7.021 = 2.25

Next step:

C = 1 X 4 = 4

H = 2.25 X 4 = 9

Now you know that answer!

It is.....: 

4 C = 48

9 H = 9