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Molecular Formula

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The molecular formula of a compound indicates the total numbers of different atoms in a molecule of it. For molecular compounds the molecular formula is determined from the empirical formula and the molecular mass of the compound.

Molecular formula of a substance is to determine the weight of one mole of that substance. Without the knowledge of the molecular mass of the unknown, there is no way of determining whether the empirical formula which is determined directly from elemental analysis, is the true formula of the substance or whether the empirical formula must be multiplied by some integral factor to obtain the molecular formula.

What is Molecular Formula?

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"The molecular formula is the true formula, representing the total number of atoms of each element present in one molecule of a compound."

It is entirely possible that two or more substances will have the same percent composition, yet be distinctly different compounds. The molecular formula is always some multiple of the empirical formula. This multiple is derived from the molecular weight of the unknown.

Molecular Formula Definition

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Definition of molecular formula

"The molecular formula of a compound may be defined as the formula which specifies the number of atoms of various elements present in the molecule of the compound."

To define molecular formula it is considered that molecular formula is a multiple of the empirical formula, this multiple is determined fro the experimental value of the molecular mass.

Molecular Weight Formula

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Molecular weight (MW) is the sum of the atomic weights of all atoms in a molecular compound expresses in atomic mass units (amu).

$Molecular\ weight = \sum Atomic\ weights$

For example the compound glucose (C6H12O6) molecular weight is expressed as

$Molecular\ weight\ of\ glucose = \sum atomic\ weights\ of\ elements$

= 6 $\times$ 12 + 12 $\times$ 1.0 + 6 $\times$ 16.0 = 180.0 amu

Molecular weight is strictly correct only when used for covalent compounds. Formula weight can be used for both ionic and covalent compounds and molecular weight only used for covalent compounds.

How to Find Molecular Formula?

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Finding molecular formula can be done by the following steps.
  1. Calculate the empirical formula.
  2. Find out the empirical formula mass by adding the atomic mass of all the atoms present in the empirical formula of the compound.
  3. Divide the molecular mass (determined experimentally by some suitable method) by the empirical formula mass and find out the value of n.
  4. Multiply the empirical formula of the compound with n so an to find out the molecular formula of the compound.


Determining molecular formula using relative data can be done by high resolution mass spectrometers. Another method of determining molecular formulas is to examine the relative intensities of the peaks due to the molecular ion and related ions.

Molecular Formula Table

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Molecular formula of the compound can be found by knowing the molecular mass of that compound. The following table summarizes the molecular formula examples for a list of compounds.

S.No
Name of the compound
Molecular formula
1 Molecular formula for butane C4H10
2 Molecular formula for glucose C6H12O6
3
Ethanol molecular formula
C2H6O
4
Molecular formula for sucrose
C12H22O11
5
Caffeine molecular formula
C8H10N4O2
6
Acetic acid molecular formula
C2H4O2
7
Molecular formula for propane
C3H8
8
Aspirin molecular formula
C9H8O4
9
Acetone molecular formula
C3H6O
10
Sugar molecular formula
C12H22O11
11
Molecular formula of water
H2O
12 Molecular formula for maltose
C12H22O11
13
Molecular formula of oxygen
O2
14
Molecular formula for methane
CH4
15
Benzene molecular formula
C6H6
16
Hexane molecular formula
C6H14
17
Methanol molecular formula
CH3OH
18
Toluene molecular formula
C7H8
19
Molecular formula for acetone
C3H6O
20
Molecular formula for ammonia
NH4
21
Molecular formula for fructose
C6H12O6
22
Salicylic acid molecular formula
C7H6O3
23
Vinegar molecular formula
CH3COOH
24
Molecular formula for ammonium nitrate
(NH4)(NO3)
25
Benzoic acid molecular formula C7H6O2
26
Sodium bicarbonate molecular formula NaHCO3
27 Sulfuric acid molecular formula H2SO4
28
Vitamin c molecular formula
C6H8O6
29
Molecular formula of starch
(C6H10O5)n
30
Gasoline molecular formula
C8H18
31
Ibuprofen molecular formula
C13H18O2
32
Molecular formula of formaldehyde
CH2O
33
Hydrochloric acid molecular formula
HCl
34
Molecular formula for glycine
C2H5NO2
35 Molecular formula of Methylene radicalCH2
36
Graphite molecular formula
C (elemental)
37
Molecular formula for glycerol
C3H8O3
38
Acetaminophen molecular formula
C8H9NO2
39
Citric acid molecular formula
C6H8O7
40
Sodium hydroxide molecular formula
NaOH
41 Molecular formula for pentane C5H12
42
Ascorbic acid molecular formula
C6H8O6

Empirical and Molecular Formula

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The molecular formula of a compound is a multiple of its empirical formula. For example, the molecular formula of acetylene is C2H2 is equivalent to (CH2)2 and the molecular formula of benzene is C6H6 is equivalent to (CH)6. Therefore the molecular mass is the some multiple of the empirical mass, which is obtained by summing the atomic masses of the atoms in the empirical formula.

For any molecular compound, we can write empirical formula to molecular formula or molecular formula from empirical formula

Molecular mass = n $\times$ empirical formula mass

Where n is the number of empirical formula units in the molecule. The molecular formula is obtained by multiplying the subscripts of the empirical formula by n, which is calculated from the equation,

$n = \frac{Molecular\ mass}{Empirical\ formula\ mass}$

Once the empirical formula for a compound is determined then empirical formula mass can be calculated. If experimental determination of molecular mass is there, then we can calculate n and then molecular formula.

Molecular Formula vs Structural Formula

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"A molecular formula shows the kind and number of atoms present in one molecule, whereas a structural formula shows in addition how the atoms are bonded together."
For example, a molecule of water that contains two atoms of hydrogen and one atom of oxygen is denoted by the molecular formula H2O and by the structural formula H-O-H. The later formula shows how one oxygen atom is bonded with two hydrogen atoms.

Molecular formula Vs structural formula interpretation is given below.

Molecular formula Structural formula
Methane - (CH4) Methane
Ethane - (C2H6) Ethane
Ethanol - (C2H6O) Ethanol

The chemical name describes the chemical structure. A chemical name is a string sequence. Since various ways of naming a compound exist, no formal definition of chemical names here. Chemical names can be single terms or phrases segmented by white spaces or punctuation marks.
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Many compounds have generic as well as chemical names. Systematic names encode the composition and structure of compounds and are based on a details set of rules established by the IUPAC. Most entries give the IUPAC name, but in some instance a more standard chemical name used by the chemical community is given.
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Chemical nomenclature is nothing but the system for naming the chemical compounds. The primary aim of chemical nomenclature is to provide methodology for assigning descriptions to chemical species so that they can be identified without ambiguity.
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A set of rules was formulated by IUPAC (International Union of Pure and Applied Chemistry) for systematic nomenclature of organic compounds which is revised from time to time.

The IUPAC name for any organic compound essentially consist of three parts.
  1. Stem name
  2. Prefix
  3. Suffix
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Chemical formula is a concise written description of the components of a chemical compound. It identifies the elements in the compound by their symbols and indicates the relative number of atoms of each element with subscripts.

Chemical formula = Parts of compound elements in symbol of atoms in subscripts
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An ionic compound is composed of cations and anions joined together. Such compounds are held together by electrostatic forces and adopt structures that maximize the attraction of oppositely charged species and minimize the repulsion between the charged species with the same sign. Ionic compound generally consist of a combination of metals with non-metals.
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Empirical formula gives the number of each type of atom in a molecule. It is defined as that formula of a substance which gives the simplest whole number ratio between the atoms of the various elements present in one molecule of the substance.
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Molecular Formula Problems

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Calculating molecular formula of a compound is illustrated by the following examples.

Solved Examples

Question 1: A compound with simplest formula which has the following percentage carbon - 80% and hydrogen - 20%. If the molecular mass is 30, calculate its molecular formula.
Solution:
 
Calculation of empirical formula

Element
Percentage
Atomic mass
Relative number moles
Simple ratio of moles
Simplest whole number ratios
80 12 $\frac{80}{12}$ = 6.66 $\frac{6.66}{6.66}$ = 1
H
20
1
$\frac{20}{1}$ = 20
$\frac{20}{6.66}$ = 3
3

Empirical formula is CH3
Calculation of molecular formula
Empirical formula mass = 12 $\times$ 1 + 1 $\times$ 3 = 15
n = $\frac{Molecular\ mass}{Empirical\ formula\ mass} = \frac{30}{15}$ = 2
Molecular formula = Empirical formula $\times$ 2 = CH3 $\times$ 2 = C2H6

 

Question 2: Find the molecular formula of a compound whose empirical formula is given as C2H4O and the vapor density of the compound was found to be 44.
Solution:
 
Empirical formula is C2H4O

Empirical formula mass = 12 $\times$ 2 + 1 $\times$ 4 + 16 $\times$ 1 = 44

Molecular mass = 2 $\times$ vapor density = 2 $\times$ 44 = 88

$n = \frac{Molecular\ mass}{Empirical\ formula\ mass} = \frac{88}{44}$ = 2

Molecular formula = Empirical formula $\times$ n = C2H4O $\times$ 2 = C4H8O2