Question

a 100 cm cube gas cylinder filled with chlorine under 160 torr pressure is connected by stop clock with another cylinder of 400 cm cube filled with nitrogen under pressure of200 torr. what will be total pressure when stop clock is opened?

Answer 1

First I want to correct one of the words in your sentences. The word is stopcock and not stop clock.
I would solve this in 4 steps.
1. Calculate n from PV = nRT for the Cl2 gas. You don't have a T listed but stick any number in there but use the same number throughout.
2. Calculate n from PV = nRT for the N2 gas.
3. Add n from 1 to n from 2.
4. Using total n, solve for P from PV = nRT. Use the same T used above and use for V the total volume of the two cylinders.
I ran through the calculation quickly and came up with approximately 200 mm (about 0.25 atm).

Answer 2

But what is the the T you kept

Answer 3

good