Samples: Multiple-Choice Questions

1. 6.0 moles of chlorine gas are placed in a 3.0-liter flask at 1250 K. At this temperature,
   the chlorine molecules begin to dissociate into chlorine atoms. What is the value for Kc
   if 50.% of the chlorine molecules dissociate when equilibrium has been achieved?

A. 1.0

B. 3.0

C. 4.0

D. 6.0

E. 12.0

Answer: C

Begin by writing the balanced equation in equilibrium.

Cl 2 (g) ↔2 Cl(g)

Next, write an equilibrium expression.

K

Cl

Cl

c

2

2

=

6

6

@

@

Then create a chart that outlines the initial and final concentrations for the various species.

Species Initial Concentration Final Concentration

Cl 2 6030 .. molesliters= 20. M 6.0 moles −(0.5)(6.0) = 3.0 moles

..
. liters

30 moles 10 M

30 =

Cl 300. litersmoles = 0 M

. moles C dissociated
mole C

moles C

1

30 1

11

(^221)
2

= 6.0 moles Cl at equilibrium
.

..
liters

moles Cl M

30

(^60) = 20
Finally, substitute the concentrations (at equilibrium) into the equilibrium expression.
.
.
K.
Cl
Cl
10
20
c^40
22
2
== =
^h
6
6
@
@
Equilibrium