Question

Hydrazine, N2H4 is a colorless, flammable liquid that is sometimes used as a rocket propellant. how many grams of hydrazine should be diluted to 250.0mL in water to create a solution with a pH=10.66?

Answer 1

N₂H₄ is weak base dissociate as given below

N₂H₄ + H₂O OH^- + N₂H₅⁺

pOH = 14 - pH = 14 - 10.66 = 3.34

[OH^-] = 10^-pOH = 10^-3.34 = 0.000457 M

[OH^-] = 0.000457 M

according to dissocition reaction

[OH^-] = [N₂H₅⁺] therefore

[N₂H₅⁺] = = 0.000457 M

K_b for weak base

Kb = [N₂H₅⁺][OH^-]/[N₂H₄]

kb of N₂H₄ = 3 X 10^-6

[N₂H₄] = [OH^-] [N₂H₅⁺] / Kb

[N₂H₄] =  0.000457 X  0.000457 / 3 X 10^-6

[N₂H₄] = 0.0696 M

no. of mole = molarity X volume of solution in liter

250 ml = 0.250 liter

no.of mole of N₂H₄ = 0.0696 X 0.250 = 0.0174 mole

to make 10.66 pH solution dissolve 0.0174 mole of hydrazine

molar mass of N₂H₄ = 32.0452 gm/mole then 0.0174 mole of N₂H₄ = 0.0174 X 32.0452 = 0.558 gm

0.558 gm of N₂H₄ of hydrazine should be dissolve in 250 ml water to make pH 10.66