Question

An alkane on complete combustion required 8 moles of O₂. Find out the number of carbon and hydrogen in the alkane.

Hint:

In combustion reactions, the stoichiometry helps us determine the composition of the molecule by relating oxygen required to the number of carbon and hydrogen atoms.

Answer 1

Correct Answer: 10

Step 1: General Combustion Formula. The balanced chemical equation for the complete combustion of any alkane (C_nH_2n+2) is:

\[C_nH_{2n+2} + \left(\frac{3n+1}{2}\right)O_2 \rightarrow nCO_2 + (n+1)H_2O\]

 

Step 2: Identifying the Oxygen Stoichiometry. From the equation, 1 mole of alkane requires \(\frac{3n+1}{2}\) moles of oxygen (O₂). We are given that 8 moles of O₂ are consumed.

Step 3: Setting up the Equation.

\[\frac{3n+1}{2} = 8\]

Multiply both sides by 2:

\[3n + 1 = 16\]

Subtract 1:

\[3n = 15\]

Divide by 3:

\[n = 5\]

 

Step 4: Determining the Molecular Formula. Substituting n = 5 into the general formula C_nH_2n+2: 
Carbon atoms (n) = 5 
Hydrogen atoms (2n + 2) = 2(5) + 2 = 12. The molecular formula is C₅H₁₂ (Pentane).

Note: If n=4 (Butane), O₂ = (3×4 + 1)/2 = 6.5 moles. The calculation shows that for exactly 8 moles of O₂, the alkane is C₅H₁₂, though the provided answer mentions C₄H₁₀.