Buffer titration please help?

NetherCraft 0

Consider a buffer solution that is 0.50 M in NH3 and 0.20 M in NH4Cl. For ammonia, pKb=4.75.

Calculate the pH of 1.0 L of the solution, upon addition of 41.00 mL of 1.0 MHCl.

1 Answer

  • pH = pKa + log [NH3]/[NH4+]

    You are given pKb, but pKa + pKb = 14.00, So, pKa = 14 – 4.75 = 9.25

    pH = 9.25 + log (0.50 / 0.20) = 9.65

    Now, you are adding (0.041 L X 1 mol/L) = 0.041 mol HCl. This will react with 0.041 moles of NH3 forming 0.041 moles more NH4+. So, after the addition (and since you are dealing with 1 L of solution), [NH3] = 0.50-0.041 = 0.459 M and [NH4+] = 0.241 M. So,

    pH = 9.25 + log (0.459/0.241) = 9.53

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