Question

Phosphine $\left(P{H}_3\right)$ decomposes to produce $P_4\left(g\right)$ and $H_2\left(g\right)$. What would be the change in volume when $100\text{mL}$ of $P{H}_3\left(g\right)$ is completely decomposed?
$4P{H}_3\left(g\right)\to P_4\left(g\right)+6H_2\left(g\right)$

• A. $50\text{mL}$
• B. $500\text{mL}$
• C. $75\text{mL}$
• D. $250\text{mL}$

Answer 1

The correct option is C $75\text{mL}$

$\begin{array}{ccccc}4P{H}_3\left(g\right)& \to & P_4\left(g\right)& +& 6H_2\left(g\right)\\ 4\text{vol.}& \to & 1\text{vol.}& & 6\text{vol.}\\ 100\text{mL}& \to & \frac{100}{4}& & \frac{6}{4}\times 100\\ 100\text{mL}& \to & 25\text{mL}& & 150\text{mL}\end{array}$

Change in volume $=(150+25)–100=75\text{mL}$
Volume will increase by $75\text{mL}$ since initially, only $P{H}_3\left(100\text{mL}\right)$ was present. After reaction $P_4\left(g\right)$ and $H_2\left(g\right)$ were formed which increased the volume.