Molecular+Formula

=Finding The Molecular Formula =

First, let's review the **Empirical Formula!** The **Empirical Formula** is the **Simplest Formula** and is the smallest whole-number ratio atoms which represents the molecular composition of a species. If the empirical formula can be found, it is easy to calculate the molar mass of the empirical formula; that is, the empirical mass.

A **molecular formula** is a way of expressing information about the atoms that constitute (make up) a particular chemical compound.

Like this picture, (From google images)

The molecular formula identifies each constituent element by its chemical symbol and indicates the number of atoms of each element found in each discrete (separate) molecule of that compound.

Let N=the WHOLE NUMBER multiple of the empirical mass multiple= N= molar mass / empirical mass

The molar mass is a mutiple of the empirical mass, then the molecular formula must be the same multiple of the empirical formula. So, remember, molecular formular = N × (empirical formula)

Let's use an example to help review.

Example: A gas has the empirical formula CH2. If 0.850 L of the gas at STP has a mass of 1.59 g, what is the molecular formula?

Answer: Firstly, empirical mass of CH2 = 12.0 g + 2(1.0 g) = 14.0 g density of gas = 1.59 g / 0.850 L = 1.87 g/L mass of 1 mol gas = (1.87 g/L) × (22.4 L/1mol)=41.90 g/mol N= molar mass / emprical mass = 41.90 g /14.0 g =2.99 ~=3 so that : molecular formula = 3 × (CH2) = C3H6

SOME PRACTISE HERE: QUESTIONS:

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Reference: all from textbook