# TopScore Exam #1 Q56

#### LoLPoPs

10+ Year Member
7+ Year Member
I'm struggling with TopScore GChem Question #56 in Exam #1.

When 14.250 moles of PCl5 gas is placed in a 3.00 liter container and comes to equilibrium at a constant temperature, 40% of the PCl5 decomposes according to the equation:

PCl5(g) <-> PCl3(g) + Cl2(g)

I can't figure out where 2.854 is coming from... I thought the bottom should be 4.75...??!?   Can someone please explain to me what's going on?
Thanks!!! #### nze82

I'm struggling with TopScore GChem Question #56 in Exam #1.

When 14.250 moles of PCl5 gas is placed in a 3.00 liter container and comes to equilibrium at a constant temperature, 40% of the PCl5 decomposes according to the equation:

PCl5(g) <-> PCl3(g) + Cl2(g)

I can't figure out where 2.854 is coming from... I thought the bottom should be 4.75...??!?   Can someone please explain to me what's going on?
Thanks!!! I'm guessing they're asking for the equilibrium constant?

Start with 14.25 moles of PCl5, 40% decomposition means 14.25*0.4 = 5.7 moles decomposed. So, the remaining number of moles of PCl5 is 14.25-5.7 = 8.55 moles. Therefore, the equilibrium molar concentration of PCl5 equals:
M = n/L = 8.55/3 = 2.85M

At the same time, since you have a 1:1:1 ratio between all components of the reaction, 5.7 moles of PCl3 and Cl2 are produced as a result of 40% decomposition of PCl5. So, the molar concentration of these two species is 5.7/3 = 1.9 (Close enough to 1.869).

Now plug these #s into the formula for the equilibrium constant and that gives you the answer.

#### LoLPoPs

10+ Year Member
7+ Year Member
I'm guessing they're asking for the equilibrium constant?

Start with 14.25 moles of PCl5, 40% decomposition means 14.25*0.4 = 5.7 moles decomposed. So, the remaining number of moles of PCl5 is 14.25-5.7 = 8.55 moles. Therefore, the equilibrium molar concentration of PCl5 equals:
M = n/L = 8.55/3 = 2.85M

At the same time, since you have a 1:1:1 ratio between all components of the reaction, 5.7 moles of PCl3 and Cl2 are produced as a result of 40% decomposition of PCl5. So, the molar concentration of these two species is 5.7/3 = 1.9 (Close enough to 1.869).

Now plug these #s into the formula for the equilibrium constant and that gives you the answer.

Ohhhhhhhhhh I get it now. Thanks so much!!! 