1) The pKa of acetic acid at 25 oC is 4.75. What is the pH of a 10-3 M solution:

Question

1) The pKa of acetic acid at 25 oC is 4.75. What is the pH of a 10-3 M solution: a) at 25 oC, b) at 50 oC? Show your work. (6 pts)

2) Which dissociation would proceed at 25 oC once the compound is added to water? Show your dissociation reactions. (12 pts)

a) NaHCO3 b) Mg(OH)2

3) Using analytical methods, determine the pH, pOH and concentration of all species for: a) 10-8 M HCl b) 10-3 M NaAc
Show all your equations and assumptions. (12 pts)

4) Using the graphical method (pC vs pH), find the pH, [HAc] and [Ac-] in a solution to which 10-3 moles of sodium acetate, NaAc, and 2 x 10-4 moles of acetic acid, HAc, pKa = 4.75, have been added per liter of solution. (25 pts)

5) Using the trial-and-error method, calculate the pH of a 10-3 M HAc following the addition of 400 mL of 10-3 M NaOH and 800 mL of 10-3 M NaOH. Estimate pH before attempting to calculate it exactly. (25 pts)

6) A 10-3 M buffer solution of NaHCO3/Na2CO3 is used to maintain pH = 10.
a) Can this solution be used for this purpose? Explain? (5 pts) -4 -4 b) What is the pH of the buffer solution after the addition of each: a) 10 M NaOH and b) 10 M HCl? (15 pts)

Explanation / Answer

1) The pKa of acetic acid at 25 oC is 4.75. What is the pH of a 10-3 M solution: a) at 25 oC, b) at 50 oC? Show your work. (6 pts)

the acid will dissociate as follows

HA <-> H++ A-

Ka = [H+][A-]/[HA]

10^-4.75 = x*x/(10^-3-x)

x = [H+] = 1.247*10^-4

pH = -log(1.247*10^-4) = 3.904

2) Which dissociation would proceed at 25 oC once the compound is added to water? Show your dissociation reactions. (12 pts)

a) NaHCO3 in water:

Na+(aq) + HCO3-(aq) and

HCO3-(aq) is more acidic than basic so

HCO3-(aq) + H2O <-> H3O+ + CO3-2(aq)

b) Mg(OH)2

this is slightly soluble

Mg(OH)2(s) <--> Mg2+ + 2OH-

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