Question

$4$ moles each of $S O_{2}$ and $O_{2}$ gases are allowed to react to form $SO_{3}$ in a closed vessel. At equilibrium $25 \%$ of $O_{2}$ is used up. The total number of moles of all the gases present at equilibrium is

• A. 6.5
• B. 7.0
• C. 8.0
• D. 2.0

Answer 1

Answer: (B)

$At \,\,t =0 \,\,\,\, SO _2+ $
$SO _2 \text { is the limiting reagent }$
$\text { Att }=0 \quad 4-x \quad 4-\frac{x}{2} \quad x$
$\Rightarrow 4-\left(\frac{x}{2}\right) \text { is the oxygen remaining }$
$\Rightarrow\left(\frac{x}{2}\right) \text { of oxygen is used up }$
$\Rightarrow \frac{x}{2}=0.25 \times 4 \Rightarrow x=2 \text { moles }$
$n_T=n_{50_3}+n_{0_2}+n_{50_2}$
$=(4-x)+4-\left(\frac{x}{2}\right)+x$
$\Rightarrow 8-\left(\frac{x}{2}\right) \Rightarrow 7 \text { moles }$