Question

A mole of pcl5 undergo thermal dissociation as pcl5 givespcl3 +cl2 ,the mole fraction of pck3 at equilibrium is 0.25and the total pressure is 2.0atm . The partial pressure of cl2 at equilibrium is ?

Answer 1

PCl5 gives PCl3 and Cl2.

thus mole fraction of PCl3 and Cl2 are equal.

thus X Cl2 = 0.25

P Of Cl2 = x * Pt

= 0.5 atm

Answer 2

Answer : The partial pressure of $Cl_2$ at equilibrium is, 0.5 atm

Solution : Given,

Mole fraction of $PCl_3$ = 0.25

Total pressure = 2 atm

The balanced thermal dissociation reaction is,

                         $PCl_5\rightleftharpoons PCl_3+Cl_2$

initially moles        1             0     0

At eqm                 (1-x)          x      x

From this we conclude that the moles of $PCl_3$ and $Cl_2$ are equal. That means the moles fraction are also equal.

Mole fraction of $PCl_3$ = Mole fraction of $Cl_2$ = 0.25

Now we have to calculate the partial pressure of $Cl_2$

$p^o_{Cl_2}=X_{Cl_2}\times P_T$

where,

$p^o_{Cl_2}$ = partial pressure of $Cl_2$

$X_{Cl_2}$ = mole fraction of $Cl_2$

$P_T$ = total pressure

Now put all the given values in the above expression, we get

$p^o_{Cl_2}=(0.25)\times (2atm)=0.5atm$

Therefore, the partial pressure of $Cl_2$ at equilibrium is, 0.5 atm