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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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nylande nylande
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11 years ago
What concentration of ammonia, [NH3], should be present in a solution with [NH4+] = 0.809 M  to produce a buffer solution with pH = 9.59? For NH3, Kb = 1.8E-5.
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#1
11 years ago
pH = pKa + log (base/acid)
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#2
11 years ago
Using the Henderson-Hasselbalch Equation:
pH = pKa + log (base/acid)
pH = pKa + log ([NH3]/[NH4+])

Kb(NH3) x Ka(NH4+) = Kw
1.8 x 10^-5 x Ka(NH4+) = 1.0 x 10^-14
Ka(NH4+) = 1.0 x 10^-14 / 1.8 x 10^-5 = 5.56 x 10^-10
pKa = - log Ka = - log (5.56 x 10^-10) = 9.26

pH = pKa + log ([NH3]/[NH4+])
9.59 = 9.26 + log ([NH3] / 0.809)
0.33 = log ([NH3] / 0.809)
0.33 = log [NH3] - log 0.809
0.33 = log [NH3] + 0.09
log [NH3] = 0.33 - 0.09 = 0.24
[NH3] = 10^0.24 = 1.74 M
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