Answer :- Oxygen, hydrogen and nitrogen gases do not react with each other at 25°C, so with the help of Dalton’s law of partial pressure
P = p1 + p2 + p3
Whereas P is the absolute pressure of the mixture.
p1 , p2 and p3 are the partial pressures of the constituent gases.
p1 = 135 mm p2 = 145 mm and p3 = 255
so that
P = 135 + 145 + 255
= 535 mm
Hence, the total pressure of the mixture = 535 mm