Q18 / 150 · MHT CET 2021 (20 Sep Shift 2) · Chemistry

Calculate change in enthalpy when $39 \mathrm{~g}$ acetylene is completely burnt with oxygen and enthalpy of combustion of acetylene is $1300 \mathrm{~kJ} / \mathrm{mol}$.

  • A
    $-975 \mathrm{~kJ}$
  • B
    $-650 \mathrm{~kJ}$
  • Correct answer
    C
    $-1950 \mathrm{~kJ}$
  • D
    $-1600 \mathrm{~kJ}$

Solution

$$ \begin{aligned} & \mathrm{C}_2 \mathrm{H}_{2(\mathrm{~g})}+\frac{5}{2} \mathrm{O}_{(2)} \longrightarrow 2 \mathrm{CO}_{(\mathrm{g})}+\mathrm{H}_2 \mathrm{O}_{(\ell)} ; \Delta_{\mathrm{r}} \mathrm{H}^{\circ}=-1300 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ & \text { Acetylene } \\ & \text { When } 26 \mathrm{~g} \text { of acetylene is completely burnt, the change in } \\ & \text { enthalpy }=-1300 \mathrm{~kJ} \\ & \therefore \text { For } 39 \mathrm{~g} \text { of acetylene, the change in enthalpy } \\ & =\frac{-1300 \times 39}{26}=-1950 \mathrm{~kJ} \end{aligned} $$

Hence, the correct answer is option: 'C'.

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