The tilt of the Earth on its axis is what causes the seasons.
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
I believe this method of problem solving would be categorized as unit or dimensional analysis.
