What is the pH of this solution?

4.00 g of hydrazine are dissolved to prepare 250.0 mL of an aqueous solution.

What is the pH of this solution?

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  • 1 month ago

    First, let's get the molarity of the solution:

    MV = mass / molar mass

    (x) (0.2500 L) = 4.00 g / 32.04 g/mol

    x = 0.49938 M

    I'm pretty sure the intent of the question writer was a 0.50 M solution, but I'll use 0.49938 and round off at the end.

    I looked up the pKb for hydrazine (it's a base) on Wiki to find 5.90. Get the Kb:

    Kb = 10^-pKb = 10^-5.90 = 1.259 x 10^-5

    Hydrazine ionizes like this:

    NH2NH2 + H2O <==> NH2NH3^+ + OH^-

    Kb = ([NH2NH3^+] [OH^-]) / [NH2NH2]

    1.259 x 10^-5 = [(x) (x)] / 0.49938

    x = 0.0007929 M <--- this is the hydroxide conc

    pOH = -log 0.0007929 = 3.10

    pH = 10.90

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