A vessel of capacity 400cc and filled with chlorine under 80 cm is connected by a narrow tube and stop with another vessel of capacity 250cc filled with nitrogen under 100cm. What will be the pressure when the stop is opened???

Answer 1

There may be an easier way to do this but here is what I would do. BTW, I assume that 80 cm is 80 cm Hg pressure.
For vessel 1 use PV = nRT. You know P, V, R, (T is not given so just plug in 273 K). Solve for n1 = number of moles gas in vessel 1.
Do the same for vessel 2 and solve for n2. Then find the total moles or
ntotal = n1 + n2 and use PV = nRT where
Psystem when stop is open, v is total volume or v1 + v2, ntotal, and plug in what you've been using for T. Solve for Psystem.
Post your work if you get stuck.