Cliffs AP Chemistry, 3rd Edition

2. 6.00 moles of nitrogen gas and 6.00 moles of oxygen gas are placed in a 2.00-liter flask
   at 500°C and the mixture is allowed to reach equilibrium. What is the concentration, in
   moles per liter, of nitrogen monoxide at equilibrium if the equilibrium constant is found
   to be 4.00?

A. 3.00 M

B. 6.00 M

C. 8.00 M

D. 10.0 M

E. 12.0 M

Answer: A

Step 1: Write the balanced equation at equilibrium.

N 2 (g) + O 2 (g) ↔2 NO(g)

Step 2: Write the equilibrium expression.

K.

NO

NO

eq^400

2

22

==

66

6

@@

@

Step 3: Create a chart that shows the initial concentrations, the final concentrations, and the
changes in concentration. Let x represent the concentration (M) of either N 2 or O 2 (their con-
centrations are in a 1:1 molar ratio) that is transformed through the reaction into NO.

Initial Change in Final

Species Concentration Concentration Concentration

N 2 3.00 M −x 3.00 −x

O 2 3.00 M −x 3.00 −x

NO 0 M +2x 2 x

Step 4: Take the concentrations at equilibrium and substitute them into the equilibrium expression.

.

K.

x

x

NO

NO

300

2

eq^400

2

2

2

22

==

=

^

^

h

h

66

6

@@

@

Step 5: Solve for xby taking the square root of both sides.

. x.

x

300

(^2200)

• =
  2 x = 6.0 −2.00x
  x = 1.50

Part II: Specific Topics