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.

A molecular formula consists of the chemical symbols for the constituent elements followed by numeric subscripts describing the number of atoms of each element present in the molecule. Molecular formulas describe the exact number and type of atoms in a single molecule of a compound. The constituent elements are represented by their chemical symbols, and the number of atoms of each element present in each molecule is shown as a subscript following that element’s symbol.

A molecular formula consists of the chemical symbols for the constituent elements followed by numeric subscripts describing the number of atoms of each element present in the molecule. Molecular formulas describe the exact number and type of atoms in a single molecule of a compound. The constituent elements are represented by their chemical symbols, and the number of atoms of each element present in each molecule is shown as a subscript following that element’s symbol.

Molecular Formula is defined as

The

Often the molecular formula is the same as the empirical formula. If the molecular formula is not the same, it will be an integral multiple of the empirical formula.

The

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Solved problems based on molecular formula are given below. ### Solved Examples

**Question 1: **A hydrocarbon contains 92.5% of carbon and 7.5% of hydrogen by weight. The molecular weight of the compound is 78. Deduce the molecular formula of the hydrocarbon.

** Solution: **

The empirical formula of the hydrocarbon = CH

The formula weight of CH = 12.011 + 1.008 = 13.019

The molecular weight of the compound = 78

We have

(Formula weight)_{n} = Molecular weight

or 13.019 n = 78

n = 6

(Empirical formula)_{n} = Molecular formula

Hence the molecular formula = (CH)_{6} = C_{6}H_{6}

**Question 2: **Empirical formula of a compound is XY_{2}. If its empirical formula weight is equal to its vapor density. Calculate the molecular formula of the compound.

** Solution: **

Molecular formula = Empirical formula $\times$ n

n = $\frac{molecular\ weight}{Empirical\ formula\ weight}$ = $\frac{2 \times vapor\ density}{Empirical\ formula\ weight}$

but Vapor density = Empirical formula weight

Therefore n = 2

Therefore Molecular formula = XY_{2} $\times$ 2 = X_{2}Y_{2}

Therefore Molecular formula of the compound = X_{2}Y_{4}

Element |
% |
atomic weight |
% atomic weight |
Relative numbers | Simplest ratio |

C | 92.5 |
12.011 |
$\frac{92.5}{12.011}$ = 7.701 |
$\frac{7.701}{7.433}$ = 0.03 | 1 |

H | 7.5 |
1.008 |
$\frac{7.5}{1.008}$ = 7.433 | $\frac{7.433}{7.433}$ = 1 | 1 |

The empirical formula of the hydrocarbon = CH

The formula weight of CH = 12.011 + 1.008 = 13.019

The molecular weight of the compound = 78

We have

(Formula weight)

or 13.019 n = 78

n = 6

(Empirical formula)

Hence the molecular formula = (CH)

Molecular formula = Empirical formula $\times$ n

n = $\frac{molecular\ weight}{Empirical\ formula\ weight}$ = $\frac{2 \times vapor\ density}{Empirical\ formula\ weight}$

but Vapor density = Empirical formula weight

Therefore n = 2

Therefore Molecular formula = XY

Therefore Molecular formula of the compound = X

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