Empirical, Molecular and Structural Formulae

Empirical Formula

The empirical formula shows the simplest whole number ratio of the atoms in a molecule. For example: the empirical formula of hydrogen peroxide is HO.

The determination of the empirical formula of an organic compound involves combustion. A known mass of the compound is burned completely in excess oxygen.

There are several crucial steps in determining the empirical formula of a compound.

Steps for determining an empirical formula

Step 1 – Start with the number of grams of each element, given in the problem. If percentages are given, assume that the total mass is 100g therefore the percent given is equal to the mass of each element present.

Step 2 – Convert the mass of each element to moles using the molar mass in the periodic table

Step 3 – Divide each mole value by the smallest mole that was calculated

Step 4 – Round off to the nearest whole number

Now lets use an example to help us understand exactly how to do this.

Question Compound X contains 45.9% carbon, 32% hydrogen 13.5% nitrogen and 8.6% oxygen. Calculate the empirical formula of compound X.

Step 1 – The mass of each element based on the question

C – 45.9g

H – 32g

N – 13.5g

O – 8.6g

Step 2 – Convert the mass to moles using the molar masses in the periodic table

C          45.9 / 12 = 3.825

H         32 / 1 = 32

N         13.5 / 14 = 0.9643

O         8.6 / 16 = 0.5375

Step 3 – Divide each mole by the smallest number of moles calculated. Remember to round off to the nearest whole number.

C          3.825/0.5375 = 7

H         32 / 0.5375 = 2

N         0.9643 / 0.5375 = 2

O         0.5375 / 0.5375 = 1

 

Empirical Formula of Compound X – C7H2N2O

 

Molecular Formula

The molecular formula shows the actual number of atoms of each element in a molecule of the compound. E.g. the molecular formula of the compound hydrogen peroxide is H2O2

Molecular formula can be determined if the molar mass of the compound is known. To find this, calculate the mass of the empirical formula and divide the molar mass of the compound by the empirical formula. Use the number calculated and multiply all the atoms by this ratio to find the molecular formula.

Using an example should make this much easier to understand.

Using the same question above:

Compound X contains 45.9% carbon, 32% hydrogen 13.5% nitrogen and 8.6% oxygen. Calculate the empirical formula of compound X. The molar mass of compound X is 482g/mol

With the empirical formula calculated to be C7H2N2O. We can now go ahead and calculate the molar mass.

C7H2N2O = (7x12g) + (2×1) + (14×2) + (16×1)

84 + 2+ 98 + 16 = 200g/mol

 

Molar Mass / empirical formula =

482g/mol / 200 g/mol = 2

 

Therefore multiply the atoms in C7H2N2O by 2

The molecular formula of C7H2N2O is C14H4N4O2

 

Structural Formula

The structural formula shows the actual number of atoms and the bonds between them; that is the arrangement of atoms in the molecule. The structural formula of hydrogen peroxide is H-O-O-H. The structural formula shows how the various atoms are bonded.

structural formula

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