CHEM 102 DWS #5 February 17, 1998
NAME(print) ,
(LAST) (FIRST)
Let QOI = (Quartet Of Information), i.e., [H1+], [OH1-] , pH and pOH.
1. Write balanced chemical equations for each of the following reactions:
A. adding perchloric acid to water.
Perchloric acid has the formula HClO4
B. adding ammonia to water.
C. adding sodium phosphate to water.
D. adding an excess of hydrochloric acid to a solution of ammonia.
E. adding methyl ammonium chloride to water.
F. adding a small amount of sodium hydroxide to
an acetic acid solution.
2. What is QOI for a 0.479 M solution of ethyl amine, C2H5NH2 ?
3. A solution is prepared by dissolving 15.8 g of potassium fluoride,
KF, in enough water to form 800 ml of solution.
What is QOI for this solution?
4. A 0.107 M solution of an unidentified organic weak base has a pH
of 8.97. What is the ionization constant for the base?
5. What is QOI for a 0.125 M solution of nitric acid, HNO3?
6. A solution is prepared by dissolving 37.8 g of triethylammonium
chloride, (C2H5)3NH1+, Cl1- MW = 107.5 g/mol) in enough water to
form 500 ml of solution. What is QOI for this solution?
7. Determine the pK value for each of the following substances:
A. acetic acid D. pyridine
B. methyl amine E. butyric acid
C. hydrofluoric acid F. ammonia
8. Determine the pH value for solutions containing each of the
substances listed in problem 7, assuming that equal molar amounts
of the weak acid or base, and its conjugate base or acid are present.
9. Determine the value of the hydrolysis constant for each of the following
A. potassium phosphate D. ammonium chloride
B. methyl ammonium chloride E. lithium acetate
C. sodium nitrite F. pyridinium iodide
ANSWERS
1. A. HClO4 is a strong acid. Need to convey that no
HClO4 is present in solution, only H1+
and ClO41- are present.
B. weak base ionization NH3 + H2O = NH41+ + OH1-
all species are present and [NH3] is very large.
C. PO43- is the strong conjugate base of the weak
acid HPO42- and it hydrolyzes
PO43- + HOH = HPO42- + OH1-
D. HCl is a strong acid, and only H1+ and
Cl1- are present. NH3 is a weak base.
The acid proton and ammonia undergo a neutralization reaction.
NH3 + H1+ ---> NH41+
E. methyl ammonium chloride, CH3NH31+, Cl1- , is a salt.
Salts are completely ionized. Inquire whether either ion
is a conjugate of a weak acid or base. ANS. CH3NH31+
is the conjugate of the weak base methyl amine, and it will
hydrolyze (i.e., react with water) as follows:
CH3NH31+ + HOH = CH3NH31+ + OH1-
F. Need to convey that sodium hydroxide is a strong base, and
because it is present in a small amount (not enough to
completely neutralize all of the acetic acid) only some
neutralization will occur. This is how buffer solutions
are prepared, i.e., by partial neutralization. The equation
for this buffer solution will be:
HC2H3O2 = H1+ + C2H3O21-
and the amounts of HC2H3O2 and C2H3O21- are comparable.
2. Solution of a weak base: B + HOH = BH1+ + OH1-
In this case B is C2H5NH2
C2H5NH2 + HOH = C2H5NH31+ + OH1-
st 0.479 lots none none
CH -x -x +x +x
@eq (0.479 - x) x x
K = 5.6E-4 = [OH1-]2/(0.479-x)
Shortcut [OH1-] = 0.0164 M; good assumption.
pOH = 1.79 pH = 12.21 and [H1+] = 6.11E-13 M
3. KF is a salt, completely ionized. F1- is the conjugate of
the weak acid HF. Hydrolysis occurs:
F1- + HOH = HF + OH1-
st 0.340 lots none none
CH -x -x +x +x
@eq (0.340 - x) x x
K = Kw/6.7E-4 = 1.49E-11 = [OH1-]2/(0.340-x)
Shortcut [OH1-] = 2.25E-6M; good assumption.
pOH = 5.65 pH = 8.35 and [H1+] = 4.44E-9 M
4. A weak base does its job by: B + HOH = BH1+ + OH1-
Knowing pH = 8.97 gives pOH = 5.03 so [OH1-] = 9.33E-6M.
But [OH1-] also equals [BH1+], and we know [B] = 0.107-[OH1-],
all leading to K = 8.14E-10.
5. Nitric acid is a strong acid, completely ionized. [H1+] = 0.125 M.
pH = 0.903 pOH = 13.10 [OH1-] = 8.0E-14
6. Triethylammonium chloride is a salt, completely ionized. The cation
is the conjugate acid of the weak base triethylamine, (C2H5)3N ,
(Kb = 5.3E-4). The cation undergoes hydrolysis:
BH1+ + HOH = B + H3O1+
st 0.352 lots none none
CH -x -x +x +x
@eq (0.352 - x) x x
K = Kw/Kb = Kw/5.3E-4 = 1.89E-11 = [H1+]2/(0.352-x)
Shortcut [H1+] = 2.58E-6 M; good assumption.
pH = 5.59 pOH = 8.41 [OH1-] = 3.88E-9 M
7. A. pKa = 4.74 D. pKb = 8.67
B. pKb = 3.36 E. pKa = 4.82
C. pKa = 3.17 F. pKb = 4.74
8. A. 4.74 D. 5.33
B. 10.64 E. 4.82
C. 3.17 F. 9.26
9. A. K = Kw/K3 D. K = Kw/1.5E-5
B. K = Kw/4.4E-4 E. K = Kw/1.8E-5
C. K = Kw/4.5E-4 F. K = Kw/2.0E-0
Send corrections, comments, suggestions to S. F. Pavkovic