Question

30. A 5.71 gram sample of PCi5 is placed in an evacuated 1.8 liter flask at 264°C and decomposes to phosphorus trichloride and chlorine gas. (a) Write the balanced equation for the decomposition reaction of PCi5. (b) Calculate the initial pressure in the flask before the PC15 decomposed, (c) If 75% of the PC15 decomposes, calculate the partial pressures of all three gases in the flask after the reaction is completed.

Answer 1

A) Balanced chemical reaction is below;

PCl5 ------> PCl3 + Cl2

B) Moles of PCl5 (n) = mass in grams/molar mass = 5.71/208.5 = 0.0274 moles

Volume (V) = 1.8 liters

Pressure (P) = ?

Temperature (T) = 264° C = 264 + 273 = 537 K

Gas constant (R) = 0.0821 L atm/mol K

Using ideal gas equation;

PV = nRT

P = nRT/V = 0.0274 * 0.0821 * 537 / 1.8 = 0.672 atm ....Answer

C)

PCl5 -----> PCl3 +    Cl2

0.672 0 0 (at t=0)

0.672-x x x (after 75% completion)

After 75% completion;

Pressure of PCl3 = Pressure of Cl2 = x

x = 0.672 * 75/100 = 0.504 atm

Pressure of PCl5 = 0.672 - 0.504 = 0.168 atm

Answer

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