a sample of pcl5 weighing 2.69 placed in 1.0 l flask and completely vaporized at a tem of 250 c the pressur at this tempreture was 1.0 atm if pcl5 dissociated are partual pressur of pcl5 pcl3 and cl2 at condition

Answer 1

Your post isn't very clear. Words omitted etc. don't help decipher the problem. BTW, it's chemistry.
2.69 what? I'll assume 2.69 grams.
mols PCl5 = g/molar mass = 2.69/about 209 = approx 0.013 but you need to be more accurate with the 209 as well as all other calculations that follow that.number, Convert mols to Pressure PCl5 using PV = nRT and P = approx 0.55.
...........PCl5 ==> PCl3 + Cl2
I............0.55............0.........0
C............-p.............p........p
E.........0.55-p...........p.........p

Then Ptotal = 1 = 0.55-p + p + p
Solve for p and 0.55-p. Remember to go through this yourself for better accuracy.