Question

5 mL of a gaseous hydrocarbon can be combusted with exactly 20mL of oxygen gas to give 15 mL of CO2. What is the number of
hydrogen atoms present in the each molecule of hydrocarbon?​

Answer 1

Given:

The volume of gaseous hydrocarbon = 5 ml

The volume of oxygen = 20 ml

The volume of CO2 produced = 15 ml

To Find:

The number of hydrogen atoms present in each molecule of the hydrocarbon.

Calculation:

- The equation of combustion of a hydrocarbon is:

CxHy + (x+y/4) O2 → xCO2 + y/2 H2O

- 5 ml of hydrocarbon = 2.23 × 10⁻⁴ moles

- 20 ml of oxygen = 8.93 × 10⁻⁴ moles

- 15 ml of CO2 = 6.7 × 10⁻⁴ moles

- Hence, the combustion of 1 mole of hydrocarbon with 4 moles of oxygen produces 3 moles of carbon dioxide.

- So, the molecule contains 3 carbon atoms, i.e., x = 3

- From the reaction:

⇒ (x+y/4) = 4

⇒ 3 + y/4 = 4

⇒ y/4 = 1

⇒ y = 4

- So, there are FOUR (4) hydrogen atoms present in each molecule of the hydrocarbon.