A 0.10 M solution of hydrazine (N₂H₄, K _b = 1.7×10^{- 6}) containing an unknown concentration of hydrazine hydrochloride (N₂H₅Cl) has a pH of 7.50. What is the concentration of N₂H₅Cl in the solution?

Question

Quizplus · Accepted Answer

Hydrazine can react with water to form hydrazine hydroxide (N₂H₄OH) according to: N₂H₄ + H2O ⇌ N₂H₄OH + OH- The equilibrium constant for this reaction is: Kw/Kb = [OH-][N₂H₄]/[N₂H₅Cl] = 1.0×10^-14/1.7×10^-6 = 5.88×10^-9 At pH=7.50, the concentration of OH- is: pH + pOH = 14 pOH = 6.50 [OH-] = 10^-6.50 = 3.16×10^-7 M The concentration of N₂H₄OH is equal to the concentration of N₂H₅Cl, since both of them formed from complete dissociation of the solute. Thus, [N₂H₄OH] = [N₂H₅Cl] = x Substituting these values in the equilibrium equation gives: 5.88×10^-9 = (3.16×10^-7)(x)/(0.10-x) Solving for x gives: x = 0.054 M Therefore, the concentration of N₂H₅Cl in the solution is 0.054 M, or choice B.

A 010 M Solution of Hydrazine (N2H4, K b = 1

A 010 M Solution of Hydrazine (N₂H₄, K _b = 1