A gaseous hydrocarbon of vapour density 14 contains 85.2% of carbon. Calculate its molecular formula.
A gaseous hydrocarbon of vapour density 14 contains 85.2% of carbon. Calculate its molecular formula.
Given: The given compound is a hydrocarbon i.e.; the given compound contains Carbon (C) and Hydrogen (H).
Step-1: Calculate the empirical formula
- Empirical formula: Empirical formula is the simplest formula of the compound which gives the simplest whole number ratio of the atoms of the various elements present in one molecule of the compound.
To calculate empirical formula % of Hydrogen is required.
As the compound consists of Hydrogen and Oxygen and it is given that Carbon is 85.2%.
Therefore, the % of hydrogen (H) = 100- 85.2%. = 14.8 %
Element % Of Element Atomic mass (g) Atomic ratio Simplest ratio Carbon (C) 85.2 12 Hydrogen (H) 14.8 1
As the simplest ratio of Carbon is 1 and Hydrogen is 2, therefore the empirical formula comes out to be CH2.
Step-2: Calculate the empirical and molecular mass
Empirical formula weight: Empirical formula weight is obtained by the addition of the atomic weight of the various atoms present in the empirical formula.
Atomic weight of Carbon= 12 u
Atomic weight of Hydrogen= 1 u
Empirical formula weight= Gram atomic weight of Carbon + 2(Gram atomic weight of Hydrogen)
Empirical formula weight= 12 +2= 14 u
Molecular formula weight: Molecular formula weight is obtained by the addition of the atomic weight of the various atoms present in the molecular formula.
When vapour density is given, molecular weight is calculated as follows:
Molecular weight= 2 × Vapour density
Molecular weight =2 x 14 = 28 u
Step-3: Calculate the molecular formula
- Molecular formula: Molecular formula is the actual formula of the compound which gives the actual number of the atoms present in the molecule of the compound.
- Relationship between empirical formula and molecular formula is given as follows: Molecular formula = n × Empirical formula, where n is an integer such as 1,2,3, etc.
As stated above, Molecular formula = n × Empirical formula, and empirical formula is CH2
Therefore, the molecular formula is (CH2)2 or C2H4.
Given: The given compound is a hydrocarbon i.e.; the given compound contains Carbon (C) and Hydrogen (H).
Step-1: Calculate the empirical formula
- Empirical formula: Empirical formula is the simplest formula of the compound which gives the simplest whole number ratio of the atoms of the various elements present in one molecule of the compound.
To calculate empirical formula % of Hydrogen is required.
As the compound consists of Hydrogen and Oxygen and it is given that Carbon is 85.2%.
Therefore, the % of hydrogen (H) = 100- 85.2%. = 14.8 %
| Element | % Of Element | Atomic mass (g) | Atomic ratio | Simplest ratio |
| Carbon (C) | 85.2 | 12 | ||
| Hydrogen (H) | 14.8 | 1 |
As the simplest ratio of Carbon is 1 and Hydrogen is 2, therefore the empirical formula comes out to be CH2.
Step-2: Calculate the empirical and molecular mass
Empirical formula weight: Empirical formula weight is obtained by the addition of the atomic weight of the various atoms present in the empirical formula.
Atomic weight of Carbon= 12 u
Atomic weight of Hydrogen= 1 u
Empirical formula weight= Gram atomic weight of Carbon + 2(Gram atomic weight of Hydrogen)
Empirical formula weight= 12 +2= 14 u
Molecular formula weight: Molecular formula weight is obtained by the addition of the atomic weight of the various atoms present in the molecular formula.
When vapour density is given, molecular weight is calculated as follows:
Molecular weight= 2 × Vapour density
Molecular weight =2 x 14 = 28 u
Step-3: Calculate the molecular formula
- Molecular formula: Molecular formula is the actual formula of the compound which gives the actual number of the atoms present in the molecule of the compound.
- Relationship between empirical formula and molecular formula is given as follows: Molecular formula = n × Empirical formula, where n is an integer such as 1,2,3, etc.
As stated above, Molecular formula = n × Empirical formula, and empirical formula is CH2
Therefore, the molecular formula is (CH2)2 or C2H4.
