Question

When 12g of carbon reacted with oxygen to form $CO$ and $C{O}_2$ at ${25}^{0}C$ and constant pressure, $77.5$ kcal of heat was liberated and no carbon was left. Calculate the mass of oxygen which reacted.
${\Delta }_{f}H\left(CO\right)=-25$ Kcal/mol.
${\Delta }_{f}H\left(C{O}_2\right)=-95$ Kcal/mol.

• A. 32 g
• B. 2 g
• C. 24 g
• D. 40 g
• E. 28 g

Answer 1

Answer: (E)

28 g

Let $x$ mol of carbon converted into $CO$, then $1-x$ mole of C converted into $C{O}_2$.

$\Rightarrow x(-25)+(1-x)\times (-95)=-77.5$

$\Rightarrow 25x+95-95x=77.5$

$\Rightarrow 17.5=70x$

$\Rightarrow x=\frac{17.5}{70}=\frac{1}{4}$

Mass of $O_2$ used $=\frac{1}{4}\times 16+\frac{3}{4}\times 32=4+24=28g$