Learning about Le Chatelier\'s Principle: For an easy 300 points: Here is the eq

Question

Learning about Le Chatelier's Principle:

For an easy 300 points:

Here is the equation for reaction of ammonia w/water:

NH3 + H20 <--> NH4+ + OH-

There were 3 mL of 0.1 M NH3 (phenolpthphalein was added):

-NH4Cl was added; solution turned clear

-HCl was added; solution became clear, gas formed

In which direction (left or right), does each reagant shift the equilibrium for the reaction of NH3 with water? Explain fully.

Also, write the net ionic equation for the reaction of NH3 with HCl.

300 points awarded to most thorough answer.

Explanation / Answer

Ammonia is a weak base so will be only partially ionized in solution

.............NH3 + H2O ? NH4+ + OH -

[initial]......0.100........... 0 ...... 0
[change].. .-x .......... +x ..... +x
[final].....0.100-x ......... x ...... x

then the equilibrium constant, Kb is given by

Kb = x^2/(0.100 - x)

now [OH-] = 1.34 x 10^-3 = x

so substitute for x and calculate Kb

To calculate pH

first find pOH = -log(1.34 x 10^-3)

then

pH + POH = 14 so

pH = 14 - pOH

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