Question

The following diagram represents the collection of CO2 and H2O molecules formed by complete combustion of a hydrocarbon. (Figure 1)
What is the empirical formula of the hydrocarbon?
Express your answer as a chemical formula.

Answer 1

Answer:

                   CH₄

Solution:

Data Give,

                       Molecules of CO₂  =  4

                       Molecules of H₂O  =  8

Assuming the masses to be equal to the number of moles produced on product side. i.e.

                       Mass of CO₂  =  176.044 g

                       Mass of H₂O  =  144.848 g

Step 1: Calculating Moles for each Element:

C  =  Mass of CO₂ × (1 mol of CO₂÷ M.Mass of CO₂) × (1 mol of C ÷ 1 mol of CO₂)

C  =  176.044 g × (1 mol of CO₂÷ 44.011 g.mol⁻¹) × (1 mol of C ÷ 1 mol of CO₂)

C =  4 mol

H  =  Mass of H₂O × (1 mol of H₂O ÷ M.Mass of H₂O) × (2 mol of H ÷ 1 mol of H₂O)

H  =  144.848 g × (1 mol of H₂O ÷ 18.106 g.mol⁻¹) × (2 mol of H ÷ 1 mol of H₂O)

H =  16 mol

Step 2: Calculate the Smallest whole number ratio as,

                                  C                                                        H

                                   4                                                        16

                                  4/4                                                    16/4

                                    1                                                         4

Result:

           Empirical Formula is CH₄

Answer 2

Answer: $CH_4$ is the empirical formula of the hydrocarbon.

Explanation: Combustion reactions are the one where hydrocarbon in the presence of oxygen decomposes to carbon dioxide and water molecule.

General equation for combustion reaction:

$Hydrocarbon+O_2\rightarrow CO_2+H_2O$

In the image, the molecule having red and black balls are carbon dioxide and the molecules having red and white balls are water molecules.

Moles of Carbon dioxide molecule are 4 which means that the there are 4 moles of carbon and 8 moles of oxygen atoms.

Moles of water molecules are 8 which means that there are 16 moles of hydrogen and 8 moles of oxygen atoms.

Thus, According to the law of conservation of mass, there should be same number of moles of each element on the reactant side as well, which gives us the molecular formula of the hydrocarbon as $C_4H_{16}$

Empirical formula is the one in which the elements in a molecule are in simplest whole number ratio. Hence, the empirical formula becomes $CH_4$.