Which of the following will give a solution with a pH>7, but is not an Arrhenius

Question

Which of the following will give a solution with a pH>7, but is not an Arrhenius base in the strict sense? Which one of the following pairs cannot be mixed together to form a buffer solution? What change will be caused by addition of a small amount of HCl to a solution containing fluoride ions (F^-) and hydrogen fluoride (HF)? The concentration of hydronium ions (H_3O^+) will increase significandy. The concentration of fluoride ions (F^-) will increase as will the concentration of hydronium ions (H_3O^+). The concentration of hydrogen fluoride (HF) will decrease and the concentration of fluoride ions (F^-) will increase. The concentration of fluoride ion (F^-) will decrease and the concentration of hydrogen fluoride (HF) will increase. The fluoride ions will precipitate out of solution as its acid salt. In which of the following aqueous solutions would you expect AgCl to have the lowest solubility?

Explanation / Answer

1.

a. NH3 is not an Arrhenius base (it doesn’t have OH)

2.

HI is a strong acid and NaOH is a strong base. After mixture, the solution will contains the excess of one of them. No buffer.

Answer: c

3. The result is H+ (from HCl) + F- = HF

Answer d

4.

Think to the common ion effect

AgCl dissociates as

AgCl = Ag+   + Cl-

in a lower proportion, if a common ion (Ag+ or Cl-) is sent in solution by a soluble salt.

In 0.02 M BaCl2   you have [Cl-] = 0.04 M, the highest (in NaCl it is 0.015M, in KCl it is 0.02 M. The common ion Ag+ has also a lower concentration in AgNO3 sol. (0.02 m).

The molar solubility of AgCl in 0.02 M BaCl2 is

[Ag+] = S = Ksp/[Cl-] = 25Ksp

Answer b

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