When a definite volume of a gaseous alkyne () is burnt completely in an excess of air, a volumetric — Mole Concept and Some Basic Concepts of Chemistry Chemistry Question
Question
When a definite volume $V$ of a gaseous alkyne ($C_nH_{2n-2}$) is burnt completely in an excess of air, a volumetric contraction equal to twice the initial volume of the alkyne is observed upon cooling the products to room temperature. Evaluate the integer value of $n$ for this specific alkyne.
💡 Solution & Explanation
Step 1: Write the balanced combustion reaction for an alkyne. $C_nH_{2n-2}(g) + \left(n + \frac{2n-2}{4}\right)O_2(g) \rightarrow nCO_2(g) + (n-1)H_2O(l)$. Step 2: Simplify the $O_2$ coefficient: $n + 0.5n - 0.5 = 1.5n - 0.5$. Step 3: Define the volume contraction equation (assuming water condenses to a liquid of negligible volume). Contraction = $V_{alkyne} + V_{O_2} - V_{CO_2}$. Contraction = $V + V(1.5n - 0.5) - Vn = V(1 + 1.5n - 0.5 - n) = V(0.5 + 0.5n)$. Step 4: Equate to the given contraction ($2V$) and solve for $n$. $V(0.5 + 0.5n) = 2V \implies 0.5 + 0.5n = 2 \implies 0.5n = 1.5 \implies n = 3$. The alkyne is Propyne ($C_3H_4$).