   Chapter 15, Problem 76SCQ

Chapter
Section
Textbook Problem

Pure PCl5 gas is placed in a 2.00-L flask. After heating to 250 °C the pressure of PCl5 is initially 2.000 atm. However, the gas slowly but only partially decomposes to gaseous PCl3 and Cl2. When equilibrium is reached, the partial pressure of Cl2 0.814 atm. Calculate Kp for the decomposition.

Interpretation Introduction

Interpretation:

The value of KP for the decomposition of phosphorous pentachloride which is placed in a 2L flask has to be calculated.

Concept Introduction:

Equilibrium constant in terms of pressure[Kp]: Equilibrium constant can be expressed in terms of partial pressures in atmospheres.

aA(g)+bB(g)cC(g)+dD(g)KP=PCc×PDdPAa×PBb

The activity of solid substance will not appear in equilibrium and numerically it is considered as one.

Explanation

Given:

PCl5(g)PCl3(g)+Cl2(g)V=2LT=250°C=523KP=2atm

Using ideal gas equation we can calculate the number of moles of PCl5

PV=nRT

n=PVRT=2×20.082×523=0.093

PCl5(g)PCl3(g)+Cl2(g)Initialpressure2atm&

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