Question

A) Does the pH increase, decrease or remain the same on the addition of the of the following?

-       A.       B.       C.    LiF to and HF solution       -       A.       B.       C.    KI to and HI solution       -       A.       B.       C.    NH4Cl to and NH3 solution       -       A.       B.       C.    HBr to an CH3COO-/CH3COOH (acetic acid/acetate buffer) solution

A. Decrease B. Remain the same C. Increase

B) Which of the following (select all that apply) gives a buffer solution when equal volumes of the two solutions are mixed?

		0.10 M HF and 0.10 M NaF
		0.10 M HF and 0.10 M NaOH
		0.50 M HF and 0.25 M NaOH
		0.25 M HCl and 0.50 M NaF

Answer 1

Question 1.

A) if we add LiF to HF solution, HF is acidic initially, when F- is added, this increase pH to a pKa value, so pH increases.

B) KI to HI has no effect, since this is strong electrolyte fo H+ remains,

C) NH4Cl and NH3 will lower the pH, since NH3 is a base, addition of NH4+ (an acid) will lower pH.

D) HBr is strong acid, by definition, will add H+, therefore decrease pH

Question 2.

A buffer solution must contain: (acid buffer) = weak acid + conjguate base (basic buffer) = weak base + conjguate acid

a) 0.10 M HF and 0.10 M NaF

this is an acidic buffer, since HF and NaF is present

b) 0.10 M HF and 0.10 M NaOH

this is not a buffer, there will be complete neutralization of acid, so only F- is left

c)0.50 M HF and 0.25 M NaOH

this will yield a buffer

0.25 M of HF remains, 0.25 M of F- will form, which is an acidic buffer

d)0.25 M HCl and 0.50 M NaF

this will form a buffer:

0.25 M of NaF will remain as F-

0.25 M of HF will form