What is the expected pH of 0.50 M NH3 (Kb = 1.8 x 10-5)?
Let us consider the reaction
NH3 + H2O ----> NH4+ + OH-
Now Kb = ([NH4+)][OH-]/[NH3] (considering [H2O] =1)
Now let us consider X mole of NH4+/OH- is formed during hydrolysis. Therefore, (0.5-X) mole of NH3 remains.
[NH4+]=[OH-]=X anf [NH3]=0.5-X
Therefore, Kb= X.X/(0.5-X)
or Kb = X2 /0.5 (X is small therefore 0.5-X is nearly 0.5)
or X = /2
X = 0.00095
Now pOH= -log[OH-] = -log[0.00095] = 3.02
therefore pH= 14-pOH = 14-3.02 = 10.98
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