The equilibrium constant (kp) for the interconversion of pcl5 and pcl3 is 0.0121: pcl5 (g) ⇌ pcl3 (g) cl2 (g) a vessel is charged with pcl5 giving an initial pressure of 0.123 atm. at equilibrium, the partial pressure of pcl3 is atm.

Let's assume that change of pressure of PCl₅ is X

According to the ICE table,
                               PCl₅(g) ⇄ PCl₃(g) + Cl₂(g)
Initial Pressure       0.123           -             -
Change                   -X               +X          +X
Equilibrium             0.123 - X       X           X

       Kp = Ppcl₃(g) x Pcl₂(g) / Ppcl₅(g)
by substitution,
0.0121 = X × X / (0.123 - X)
0.0121 × (0.123 - X) = X²
1.4883 × 10⁻³ - 0.0121X = X²
X² + 0.0121X - 1.4883 × 10⁻³  = 0
X₁ = 0.03735 or X₂ = -0.03865
X cannot be a negative value since X is a pressure.
Hence,
   X = 0.03735 atm

Hence, partial pressure of PCl₃ is 0.03735 atm

Outermost (valence) electrons