calculate Ka for 0.10 M NH4Cl (pH= 4.86) ?
pH = 4.86
pH = -log [H+]
4.86 = -log [H+]
[H+] = 1.38 10-5 M
NH4+
NH3 + H+
The equilibrium constant for this weak acid is:
Ka = [NH3] [H+] /
[NH4+]
Using ICE, we would get for the equilibrium (i.e. final)
concentrations (let x = [H+]):
[NH4+] = 0.1 - x (initial 0.1 M, change
-x)
[NH3] = x (initial 0, change + x)
[H+] = x (initial 0, change + x)
Ka = x2 / 0.1-x
0.1>>>x, so we can write 0.1 instead of 0.1-x
Ka = x2 / 0.1 = (1.38 10-5)2/0.1 = 1.90 10-9
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