Question

Calculate the work done during combustion of $0.138 \,kg$ of ethanol, $C_2H_5OH_{(l)}$ at $300\, K$.
Given : $R = 8.314 \, Jk^{-1} mol^{-1}$, molar mass of ethanol = $46 \, g \, mol^{-1}$.

• A. -7482 J
• B. 7482 J
• C. -2494 J
• D. 2494 J

Answer 1

Answer: (B)

The combustion of ethanol, involves the following reaction.

$C_2H_5OH(l) +3O_2 (g) \to 2CO_2+3H_2O$

Given,

Mass of ethanol $= 0.138 \,kg = 138 \,g$

Temperature = $300 \,K$

$R=8.314\,J\,K^{-1}\,mol^{-1}$

Molar mass of ethanol $=46\,g\,mol^{-1}$

No. of moles of ethanol = $\frac{\text { Mass of ethanol }}{\text { Molar mass of ethanol }}$

$=\frac{138}{46}=3$

Work done $(W)$ during combustion of $0138 \,kg$ of $C _{2} H _{5} OH =n R T $

$W =n R T $

$W =3 \times 8.314 \,JK ^{-1} \,mor ^{-1} \times 300\, K $

$W =7482.6\, J$

$ \approx 7482\, J$