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Kc and pH chem problem?
A solution of hydrazine, N2H4, has a concentration of 0.25 mol.dm^-3. What is the pH of the solution, and what is the percentage ionization of hydrazine? Hydrazine has a Kb=1.7*10^-6.
1 Answer
- gp4rtsLv 78 years agoFavorite Answer
N2H4 + H2O <----> N2H5+ + OH-
Kb = [N2H5+]*[OH-]/[N2H4]
Initial concentrations:
N2H4 = 0.25 M
[OH-] = 0
[N2H5+] = 0
Change if x moles of N2H4 react
∆N2H4 = -x
∆[OH-] = +x
∆[N2H5+] = +x
Equilibrium
N2H4 = 0.25 - x
[OH-] = x
[N2H5+] = x
Kb = x²/(0.25 - x)
Because Kb is small, we expect x to be small, and then 0.25 - x ≈ 0.25
x = √[1.7*10^-6*0.25] = 6.52*10^-4
[OH-] = x = 6.52*10^-4
pOH = 3.186
pH = 14.0 - pOH
pH = 10.81
fractional ionization = x/0.25 = 0.00266
percent ionization = 0.27%