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Determining Equilibrium Quantities
from Initial Quantities and K

To find the equilibrium quantities of each species from the initial quantities we must know:

Once these have been determined, we can solve for the equilibrium concentrations using the following steps: Determining Equilibrium Concentrations

 Example:  0.050 mol of H2 and 0.050 mol of Br2 are placed in an evacuated 5.0 L flask and heated to 700 K.  What is the concentration of each species in the flask when equilibrium has been established?  The equation for the reaction is as follows:

H2(g) + Br2(g) double arrows  2 HBr(g)          Kc = 64 @ 700 K

Equilibrium Expression
           [H2] = 0.050 mole H2/5.0 L = 0.010 M
            [Br2] = 0.010 M
            [HBr] = 0 M
 
H2
Br2
HBr
Initial Concentration (M)
0.010
0.010
0
Change in Concentration (M)
- x
- x
+ 2 x
Equilibrium Concentration (M)
0.010 - x
0.010 - x
0 + 2 x
Equilibrium Solution

x = 0.008 M

Checking Answers

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Determining Equilibrium Pressures

Example:  1.000 atm of SO3, 0.150 atm of SO2, 0.200 atm of NO2, and 2.000 atm of NO at 460oC was allowed to reach equilibrium.  What is the equilibrium pressure of each gas present in the flask?

SO2(g) + NO2(g) double arrows  NO(g) + SO3(g)     Kp = 85.0 @ 460oC

K<sub>p</sub> Expression
Solving for Q

 Kp < Q so the reaction will proceed in the reverse direction.

Solving for x

        x = 0.013  atm

PSO2 = 0.150 + x = 0.150 + 0.013 = 0.163 atm
PNO2 = 0.200 + x = 0.200 + 0.013 = 0.213 atm
PNO = 1.500 - x = 1.500 - 0.013 = 1.487 atm
PSO3 = 2.000 - x = 2.000 - 0.013 = 1.987 atm
Checking Answers

which equals 85.0 within the uncertainty of the calculation.

For more information on the mathematics employed in solving equilibrium problems, click here.

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