Molecular Formula | Definitions, Examples

Definition of Molecular Formula

Molecular formula indicate the simple numbers of each type of atom in a molecule of a molecular substance. They are the same as empirical formulae for molecules with only one atom of a particular type but otherwise may have larger numbers.

molecular formula

An example of the difference is the empirical formula for glucose, which is CH2O (ratio 1:2:1), while its molecular formula is C6H12O6 (number of atoms 6:12:6). For water, both formulae are H2O.

A molecular formula provides more information about a molecule than its empirical formula but is more difficult to establish.

Other Definitions

Here are some more definitions related to molecular formulae:

Definition-1 (molecular formula)

A molecular formula shows the number of elements in a molecule and determines whether it is a binary compound, ternary compound, quaternary compound, or has even more elements.

And for each compound, they all have a molecular formulae. But some can be similar, and those are called isomers, which are common in organic chemistry.

Definition-2

When the molar mass value is known, the Molecular Formula is calculated by the empirical formula.

n=molar mass/mass of the empirical formula

The molecular formula is often the same as an empirical formula or an exact multiple of it.

Definition-3

It is a shortened (abbreviated) way of describing molecules and compounds. Special notations are used to indicate what elements make up molecules and compounds and what atoms make up elements.

All chemists are well versed in understanding molecular formulas to perform their jobs. The symbols for the elements are found in the periodic table, which was developed around 1870 by Russian chemist Dmitri Mendeleev (1834 –1907).

The conventional form for writing a molecular formulae is to write the symbol for each element followed by a subscript indicating the actual number of those atoms present in a molecule. When only one atom of an element is present, the subscript is omitted.

Definition-4

A molecular formulae consists of the chemical symbols for the constituent elements followed by numeric subscripts. Which describing the number of atoms of each element present in the molecule.

Definition-5

The empirical formula represents the simplest whole-integer ratio of atoms in a compound. The molecular formulae can be the same as or a multiple of the compound’s empirical formulae.

Summary of Molecular Formula

Molecular formulas describe the exact number and type of atoms in a single molecule of a compound. Their chemical symbols represent the constituent elements, and the number of atoms of each element present in each molecule is shown as a subscript following that element’s symbol.

The molecular formulae expresses the proportions of atoms that constitute a particular chemical compound, using a single line of chemical element symbols and numbers.

It also includes other symbols, such as parentheses, dashes, brackets, plus (+) and minus (–) signs.

For organic compounds, carbon and hydrogen are listed as the first element in the molecular formulae, and the remaining elements follow them in alphabetical order.

example: For butane, the molecular formula is C4H10.

For ionic compounds, the cation precedes the anion in the molecular formula. For example, the molecular formula of sodium fluoride is NaF.

A molecular formula is not a chemical name, and it contains no words. Although a molecular formulae may imply certain simple chemical structures. But it is not the same as a full chemical structural formula. Molecular formulas are more limiting than chemical names and structural formulas.

The molecular formula for glucose is C6H12O6. The molecular formulae indicates the exact number of atoms in the molecule.

A molecular formula represents a molecule that uses chemical symbols to indicate the types of atoms followed by subscripts to show the number of atoms of each type in the molecule. (A subscript is used only when more than one atom of a given type is present.) The molecular formulas are also used as abbreviations for the names of compounds.

How to Culculate the Molecular Formula ?

calculate molecular formula

Here are some solved examples for calculating the molecular formula

Example-1

The empirical formula of hexane is C3H7. Its molecular weight is 86.2 amu. What is the molecular formula of hexane?

Solution: The molecular formula of a compound is a multiple of its empirical formulae. The molecular formulae weight is some multiple of the empirical formula weight.

We know the empirical formula so we can calculate the empirical formula weight. We can calculate what multiple the molecular formulae weight is of the empirical formulae weight. Because the molecular formulae is the same multiple as the empirical formula.

  1. Calculate the formula weight of C3H7.

Carbon 12.01×3 = 36.03

Hydrogen 1.008×7 = 7.056

2. Calculate the ratio between the molecular weight and the empirical weight:

Molecular weight/ Empirical weight = 86.2/43.09 = 2.0005

3. The molecular formula must be twice the empirical formula:

(C3H7)2 or C6H14.

So the molecular formulae of Hexane is, (C3H7)2 or C6H14.

Example-2

The compound ethylene glycol is also used as an antifreeze. It contains 38.7% carbon, 9.75% hydrogen, and the rest oxygen. The molecular weight of ethylene glycol is 62.07g. What is the molecular formulae of ethylene glycol?

Solution:

  1. Calculate the empirical formula. Assume 100g of the compound, which will contain 38.70 g carbon, 9.75 g hydrogen and the rest oxygen?

gO = 100 g – 38.70, gC – 9.75g, H = 51.55 gO.

2. Calculate the moles of each element present:

?g mol C = 38.70g C x 1 mol C/ 12.01g C = 3.22 mol C

?gmol H = 9.75g H x 1 mol H / 1.008g H = 9.67 mol H

?g mol O = 51.55g O x 1 mol O / 12.01g O = 3.22 mol O

Next calculate the ratio of molecular weight to empirical formulae weight. The molecular weight is given. And the empirical formula is CH3O, so the empirical formula weight is 12.01 + 3(1.008) + 16.00 = 31.03.

Molecular weight / empirical formulae weight = 62.07 / 31.03 = 2

Therefore the molecular formula is twice the empirical formula: C2H6O2.

Example-3

The compound dioxane contains only carbon, hydrogen, and oxygen. When 0.956g dioxane is burned, 1.91g carbon dioxide and 0.782g water are formed. In another experiment, it was determined that 6.04×10-3 mol dioxane weighs 0.532g. What is the molecular formulae of dioxane?

Solution:

  1. Calculate the molecular weight of dioxane.

Molecular weight = grams/mole = 0.532g dioxane / 6.04 x 10-3 = 57.4 88.1 g/mol

2. Calculate the molecular formula of dioxane.

The empirical formula weight is 2(12.0) + 4(1.01) + 16.0 = 44.0. The molecular weight is 88.08. The ratio of molecular weightto empirical weight is:

Molecular weight/empirical weight = 88.0/44.0 = 2

So the molecular formula is: C4H8O2.

Example-4

A compound contains 86.88% of carbon and 13.12% of hydrogen, and the molecular weight is 345. What will be the empirical and molecular formulae of the compound?

Solution:

Step-1: Carbon : 345 x 0.8688 = 299.736

Hydrogen : 345 x 0.1312 = 45.264

Step-2: Carbon : 299.736 / 12.011 = 24.995

Hydrogen : 45.264 / 1.008 = 44.91

Step-3: Molecular formula is C25H45.

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