Question #6fe0b
1 Answer
Explanation:
Let's first write the chemical equation for this reaction:
And so the equilibrium constant expression is
Let's make a list of our initial concentrations (no volume is given, but this doesn't matter as we're dealing just with quantity):
Initial:

#"PCl"_5# :#3# #"mol"# 
#"PCl"_3# :#3# #"mol"# 
#"Cl"_2# :#2# #"mol"#
Using the coefficients of the chemical equation (which are all
Change:

#"PCl"_5# :#x# 
#"PCl"_3# :#+x# 
#"Cl"_2# :#+x#
[I'll make the assumption that the reaction quotient
Since at equilibrium, there are
Which means the final concentrations are

#"PCl"_5# :#3# #"mol"# # 1.5# #"mol"# #= 1.5# #"mol"# 
#"PCl"_3# :#3# #"mol"# #+ 1.5# #"mol"# #= color(red)(4.5# #color(red)("mol"# 
#"Cl"_2# :#2# #"mol"# #+ 1.5# #"mol"# #= 3.5# #"mol"#
Thus, at equilibrium, there will be
(You can even experiment the situation with different volumes to prove that you'll get this same value every time, no matter the volume.)