Chem 1721/1821
Name _______________________________
Quiz 6 (25 points)
April 3, 2014
pX = −log X
pKw = pH + pOH
!=
1.
Kw = 1 x 10—14 at 25ºC
pKw = pKa + pKb
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[!"#$]
Indicate whether each of the following ions is acidic (A), basic (B) or neutral (N) in aqueous solution at 25°C:
SO42– ________________
2.
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ClO4– _______________
Consider 0.150 M solutions of the following at 25°C, and put them in order of increasing pH:
LiCl, CsOH, NH4NO3, HBr, Na3PO4
______________ < ______________ < ______________ < ______________ < ______________
lowest pH
highest pH
3.
a.
For hypochlorous acid pKa = 7.46. Calculate Kb for the hypochlorite ion.
b.
Calculate the pH of 1.35 M NaOCl (aq).
Kb = _______________________
pH = ________________
c.
4.
5.
Now consider that a small amount of NaOCl is added to a solution of HOCl (aq). How do the equilibrium position,
pH, and percent dissociation of the solution change when NaOCl is added? Circle the one best answer for each.
i.
the equilibrium position will:
shift to the left
shift to the right
not change
ii.
pH of the solution will:
increase
decrease
remain the same
iii.
% dissociation of HOCl will:
increase
decrease
remain the same
a.
Write the balanced net ionic equation for the equilibrium that shows why an aqueous solution of the salt pyridinium
nitrate (C5H5NHNO3) has a pH < 7.0. Include all charges on ions.
b.
Write the balanced net ionic equation for the neutralization reaction that occurs when a small amount of strong base
(OH–) is added to a buffer solution composed of NH3 and NH4Cl. Include all charges on ions.
Consider a buffer solution composed of a weak base (B) and its conjugate acid (BH+). If a small amount of strong base is
added to this buffer solution, how will [BH+] and the pH of the solution change? Circle the one best answer for each.
a.
[BH+] will:
b.
pH of the solution will:
increase
increase slightly
decrease
decrease slightly
remain the same
remain exactly the same
6.
True or False: For a buffer solution, pH = pKa only when [base] = [acid] = 1.00 M.
________________
7.
Consider a buffer solution prepared by mixing equal volumes of 0.650 M NaHC2O4 (aq) and 0.594 M Na2C2O4 (aq).
a.
Identify the acid and base components in this buffer solution:
acid ________________ base ________________
b.
If the pH of this buffer solution is 4.171 before the addition of any strong acid or strong base, determine the pKa of
HC2O4–.
pKa = ________________
c.
Now consider that 0.150 L of 0.280 M HCl (aq) is added to 0.200 L of this buffer solution. Determine the
following: total solution volume after addition of HCl, mol H+ added, and the change in pH of the
solution that results from this addition.
total sol'n volume = _________________ mL
mol H+ added = _________________
∆pH = _________________
8.
Consider the following list of acids. Circle the one that you would choose to prepare a buffer solution with pH = 8.5.
HCN (pKa = 9.21)
HC3H5O2 (pKa = 4.89)
H2NNH3+ (for H2NNH2, pKb = 5.52)
9.
True or False: Before the stoichiometric point in a strong acid + strong base titration, a buffer solution is formed as a
result of the neutralization reaction.
10.
C5H5NH+ (for C5H5N, pKb = 8.77)
________________
35.00 mL of 0.6249 M HNO3 (aq) is titrated with 0.8011 M KOH (aq).
a.
Determine the pH of the solution after the addition of 15.00 mL KOH (aq).
pH = ________________
b.
Determine the pH of the solution after the addition of 35.00 mL KOH (aq).
pH = ________________