Question

The combustion of hydrogen-oxygen mixture is used to produce very high temperatures $(={2500}^{\circ }\text{C})$ needed for certain types of welding operations.
$H_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to H_{2}O\left(g\right)$; $\Delta H=-230.9\text{kJ}$
Quantity of heat, (in kJ) evolved when a $200\text{g}$ mixture containing equal parts of $H_2$ and $O_2$ mass is burned, is

• A. $-2.72\times {10}^3\text{kJ}$
• B. $-1.36\times {10}^3\text{kJ}$
• C. $-1.44\times {10}^3\text{kJ}$
• D. $-4.3524\times {10}^4\text{kJ}$

Answer 1

The correct option is C $-1.44\times {10}^3\text{kJ}$

Total mass $=200\text{g}$
$O_2$ taken $=100\text{g}=\frac{100}{32}=3.125\text{mol}$
$H_2$ taken $=100\text{g}=\frac{100}{2}=50\text{mol}$
$O_2$ is the limiting reagent
$H_2$ used in combustion $=6.25\text{mol}$
Thus, heat evolved $=-230.9{\text{kJ mol}}^{-1}\times 6.25\text{mol}$
$=-1443.125\text{kJ}=-1.44\times {10}^3\text{kJ}$