Question

The $P,V$ measurements for $100g$ of phosgene gas at ${25}^{\circ }C$ are given below:

$Patm$	$Vd{m}^3$	$PVatmd{m}^3$
$0.1$	$244.5$	$24.45$
$0.4$	$61.02$	$24.42$
$2.0$	$12.17$	$24.34$
$8.0$	$2.925$	$23.40$
$9.8$	$0.020$	$0.20$
$20.0$	$0.020$	$0.40$

Why did the volume of phosgene remain constant at $0.020d{m}^3$ when the pressure is increased from $9.8atm$ to $20.0atm$?

Answer 1

Increase in volume with increase in pressure at constant temperature means that it follows Boyle's law initially. At high pressures gases tend to not follow boyle's law.
When pressure changes from 9.8 atm to 20.0 atm , the gas undergoes phase change. Phase change is the phenomenon of changing the state of any substance from solid to liquid or liquid to vapor, etc.

At the time of changing phase, the volume remains same. Once it has completely changed its phase, the volume changes significantly.
This is why volume during pressure 9.8 atm and 20 atm remains same.