Question 24 TK02 - Tank Vessel Assistant - Liquefied Gas

The Correct Answer is C ### Why Option C ("heavier") is correct: Liquefied petroleum gas (LPG) is primarily composed of propane ($\text{C}_3\text{H}_8$) and butane ($\text{C}_4\text{H}_{10}$) vapors. To determine if these vapors are heavier or lighter than air, we compare their molecular weights to the average molecular weight of air. * **Average Molecular Weight of Air:** Air is a mixture mainly of nitrogen ($\text{N}_2$, molecular weight $\approx 28$) and oxygen ($\text{O}_2$, molecular weight $\approx 32$). The average molecular weight of air is approximately $29 \text{ g/mol}$. * **Molecular Weight of LPG Vapors:** * Propane ($\text{C}_3\text{H}_8$): Molecular weight $\approx 44 \text{ g/mol}$. * Butane ($\text{C}_4\text{H}_{10}$): Molecular weight $\approx 58 \text{ g/mol}$. Since both propane and butane have molecular weights significantly greater than the average molecular weight of air ($44 > 29$ and $58 > 29$), LPG vapors are denser and therefore **heavier** than air. This characteristic is critical for safety, as leaking LPG vapors will sink and accumulate in low-lying areas (such as basements or ditches). ### Why the other options are incorrect: * **A) lighter:** This is incorrect because the individual components of LPG (propane and butane) have molecular weights substantially higher than the average molecular weight of air. Lighter gases, such as methane (natural gas), would dissipate upwards, but LPG sinks. * **B) the same:** This is incorrect. For the weights to be the same, the molecular weight of the gas would have to be approximately $29 \text{ g/mol}$. LPG's components range from $44 \text{ g/mol}$ to $58 \text{ g/mol}$, making them much heavier. * **D) variable:** While the exact composition of LPG can vary slightly (more propane in colder climates, more butane in warmer climates), both propane and butane are consistently much heavier than air. Therefore, the weight comparison (heavier than air) is not variable.