Question

When 12 gm of carbon reacted with oxygen to form $CO$ and $C{O}_2$ at ${25}^{\circ }C$ at constant pressure, 60.25 kcal of heat was liberated and no carbon remained. Calculate the mass of oxygen needed for this reaction.
$\Delta H_f\left(C{O}_2\right)=-94kcal/mol$
$\Delta H_f\left(CO\right)=-26.5kcal/mol$

• A. 26.6
• B. 24
• C. 28.6
• D. 25

Answer 1

The correct option is B 24

$C+O_2\to C{O}_2;\Delta H_f=-94kcal$
$C+\frac{1}{2}{O}_2\to CO;\Delta H_f=-26.5kcal$
Let the moles of carbon that react to form $C{O}_2$ and $CO$ be 'a' and 'b'.
Since 12 gm or 1 mole carbon is used up
$a+b=\frac{12}{12}=1\dots \left(1\right)$
$a\times 94+b\times 26.5=60.25$
$a=0.5$ i.e. mole of $C{O}_2$ formed is $0.5$
$b=0.5$ i.e. mole of $CO$ formed is $0.5$
Also moles of $O_2$ used for this change
$=0.5+\frac{0.5}{2}=0.75$
Thus, weight of $O_2$ used $=0.75\times 32=24gm$