Question

1.) You have 100.0mL of 0.100 M aqueous solutions of each of the following acids: pyridine, NaOH, CH3NH2, and NaCN. You titrate each with 0.100 M HCl (aq). Rank the pHs of each of the solutions when each are titrated to the equivalence point, from highest to lowest pH. PLEASE SHOW ALL WORK

A) pyridine, NaCN, CH3NH2, NaOH

B) NaOH, CH3NH2, NaCN, pyridine

C)NaOH, CH3NH2, pyridine, NaCN

2.) The Ksp for Ag2SO4 is 1.2x10^-5 . When 40 mL of 0.020M Na2SO4 is mixed with 40 mL of 0.045 M AgNO3, will a precipitate form? PLEASE SHOW ALL WORK

A) No, because Q is 5.1x10^-6 and since it is less than Ksp no precipitate will form

B) Yes, because Q is 5.1x10^-6 and since it is less than Ksp a precipitate will form

C) No, because Q is 4.1x10^-5 and since it is more than Ksp no precipitate will form

Answer 1

lets see the reactions with

a) pyridine

Py + Hcl -------> PyHCl a salt of weak base and strong acid , is formed which is acidic and pH <7

pH = 1/2[pKW -PkB -logC]

= 1/2[ 14-8.75 l-log 0.10/200]

= 4.2755

b) NaOH + HCl ----------------> NaCl + H2O , a salt of strong acid and strong base is formed, which is neutral with pH=7

c) CH3NH2

CH3NH2 + HCl ---------------> CH3NH3Cl

A salt of weak base and strong acid , which is acidic is formed with pH <7

pH = 1/2[ 14-3.36 -log0.1/200]

= 6.97

d) NaCN

NaCN + HCl ---------------> HCN + NaCl

Now a weak acid is formed which has pH < 7

pH = 1/2 [pKa-logC]

=1/2[9.21-log 0.1/200]

= 6.255

The ranking we can decide by the pKb and pKa values of weak bases and weak acids .

PKa of HCN= 9.21

pkb of pyridine = 8.75

pkb of CH3NH2 = 3.36

thus the randking from highest to lowest

NaCl > CH3Nh2 > NaCN > pyridine

Q2)

{So4-2] in the mixture = 40x0.02/80 = 0.01 M

[Ag+] in the mixture = 40x0.045/80 = 0.0225 M

Thus

Ag2SO4 (s) -------------->2 Ag+ + So4-2

Ksp = Q = [Ag+]²[So4-2]

= (0.0225)²(0.01)

=5.06x10^-6

Q is less than Ksp , therefore no precipitate will form.

OPTION A