Chemistry 125126
Hourly II review notes and questions to check your understanding.
N. Kerner
Hourly II page 1/5
Chem. 125/126 Review E4W08
E4 Acids, Bases and Salts
1. Acid-base strength, pH and % ionization(Part 1A)
- The lower (higher) the pH of equimolar acids (bases) the stronger the acid (base)
pH = -log [H3O+]; [H3O+] [OH-] = 1 x 10-14; acids = pH < 7; bases = pH >7
- Strong (weak) acids/bases ionize 100% (<100%);% ionization can be determined from pH
% ionization acid = [H3O+]/[acid]i x 100%; % ionization base = [OH-]/[base]i x
100%
2. Acid-base props of compounds with element & O & H(XOaHb compounds, part 1B):
- are related to the position of X in the periodic table
X = metal, compound is base (ex. NaOH); X = nonmetal, compound is acid (ex. HNO3)
- are related to the electronegativity of X
The higher (or lower) the electronegativity of X the more acidic (more basic) the compound.
Examples:
1. H3PO3 (electronegativity P = 2.1) stronger acid than H3BO3 (electronegativity B = 2.0) and
thus pH H3PO3 lower than pH H3BO3 for equimolar solutions.
2. NaOH stronger base (electronegativity Na = 0.9 ) than Al(OH)3 (electronegativity Al = 1.5)
and thus pH NaOH higher than pH Al(OH)3 for equimolar solutions.
3. Conjugate Acids and Bases (Part 2)
- Strength of acids (bases) and their conjugate bases (acids) are inverse.
Observed: pH HCN higher than pH HCl for equimolar solutions and therefore
Acid Strength: HCl > HCN
∴ can predict Base strength of conjugates: CN- > Cl- Conjugate strength is predictable given the strength of its conjugate pair
Examples:
1. pH tests indicate HCN is a weak acid and ∴ CN- is a weak conjugate base
2. pH tests indicate HCl is a strong acid and ∴ Cl- is a very weak conjugate base
note: Cl- ion is such a weak base that it is, in effect, a spectator ion.
4. Acid-Base Neutralization and Indicators (Part 3)
- the salt product of acid-base neutralization is not necessarily neutral.
(e.g. NaCl from HCl + NaOH is neutral; NaC2H3O2 from HC2H3O2 + NaOH is basic)
- Properties of salt can be predicted from strengths of reacting acid and base.
Examples:
1. Strong acid + strong base = neutral salt (conjugate base & acid product are spectators)
HCl
+ NaOH
= NaCl
(Cl- and Na+ are spectator base & acid ions)
2. Weak acid + strong base = basic salt (weak conjugate base; conjugate acid is spectator)
Hourly II page 2/5
Chem. 125/126 Review E4W08
HCN
+ NaOH = NaCN (CN- = weak base; Na+ = spectator ion)
3. Strong acid + weak base = acidic salt (conjugate base is spectator; conjugate acid is weak)
HCl +
NH3
= NH4Cl (Cl- is spectator; NH4+ is weak conjugate acid)
- Indicator choice effects titration results.
- the chosen indicator end point pH should = the equivalence point pH.
note: the pH of salt and water products = equivalence point pH.
For example, indicator should have end point pH ≅ 7 if salt product is neutral
(e.g.NaCl). Indicator end point pH should ≅pH 8-9 if salt product is a weak base
- The higher (lower) the indicator pH end point, the more (less) titrant
(e.g. NaOH) will be required to reach the end point (indicator color change)
Ex.Class data shows greater volume (mL) NaOH used titrating acetic acid with
phenolpthalein (end point pH 9.1) than bromophenol blue indicator (end point pH 3.8)
5. Equivalent Weight Titrations (Parts 4-5)
Experimental Equivalent Wt = g sample
OR Theoretical Equivalent Wt. = Molecular wt
mol of titrant
# ionizable H
EXPERIMENT 4 sample Questions
Part 1 Structure and Acid-Base Properties
1.
A. Based on your knowledge of chemical periodicity, predict the relative acid strength of
H2SO4, H3PO4, HClO4, and Mg(OH)2.
weakest acid
strongest acid
_______ < ________
< ________
<_________
B. The electronegativity of phosphorous (P) is greater than the electronegativity of arsenic (As).
Predict the comparative pH of 0.10M H3PO4 and 0.10M H3AsO4.
Indicate (circle) if the pH of 0.10M H3AsO4 is
Less than
equal to
greater than
pH H3PO4 (pH 1.5)
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Chem. 125/126 Review E4W08
2.
Below are observed pH readings for a group of acids:
A. Based on the pH data, rank the acid strength:
0.10M acid
pH
Conjugate Base
HCN
4.89
HCl
1.00
HC7H5O2
2.96
Acid Strength
------------> ------------> ----------
(try April’05, 1B,1D; Dec.05, 1A. 3A; April’06, 1B, 2A; Dec.06, 2A; April’07, 1A, 3A;
Dec.07, 1 (1 & 2), 2A)
Part 2. Conjugate Acid-Base Pairs
2.
B. Complete the above table (column 3) by filling in the conjugate base formula of each acid.
C. Based on the above pH data which 0.10 M salt solution has the highest pH:
NaCl, NaC7H5O2, or NaCN ?
The salt with the highest pH =
3.
Below are observed pH readings for 0.10M solutions of sodium salts:
sodium acetate = pH 7.4; sodium benzoate = pH 8.4; sodium lactate = pH 8.0
Predict the comparative acid strength of benzoic, lactic, and acetic acids.
strongest acid
_______
weakest acid
>
________
>
________
(try April’05 2(1,2,3,4); Dec.05, 1C, 3B; April’06, 1A. 2B; Dec.06, 2B; April’07, 3B, 3C; Dec.’07, 2B)
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Chem. 125/126 Review E4W08
Parts 3-4. Acid-Base Neutralization & Indicator Color Changes
4.
You perform several titrations (listed below) using 0.01M NaOH. For each titration:
− give the formula of the salt that will be formed at neutralization
- indicate if the salt and water product will be acidic, basic, or neutral
− choose the best of four available indicators for each titration.
Available Indicators pH range where indicator changes
A. Metanil yellow
1.2-2.4
B. Ethyl red
4.0-5.8
C. Litmus
6.0-8.0
D. Thymol blue
8.0-9.6
0.01M Acid
acid pH Salt /acidic,basic,neutral?
CH3CH2COOH
2.9
HNO3
2.0
5.
Indicator to use (A, B, C, or D)
You titrate 0.193 g of an acid unknown with 0.10 M NaOH and 32.99 mL of base is
needed to reach the end point. Your calculated equivalent weight for the acid is 58.50.
The titration was performed using phenolpthalein (pH color change interval = 8.2-10.0).
You repeat the above titration (i.e. you titrate 0.193 g of the unknown acid) but use methyl
orange indicator (pH color change interval = 3.2-4.4) . Determine the effect, if any, on:
A. The volume of 0.10M NaOH required to reach the end point.
B. The calculated equivalent weight of the acid.
A.
Indicate (Circle) the volume of 0.10M NaOH used at the end point.
more than 32.99mL
B.
Equal to 32.99mL
less than 32.99mL
Indicate (Circle) the effect of the result on the calculated equivalent weight of the acid
more than 58.50
equal to 58.50
less than 58.50
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Chem. 125/126 Review E4W08
Part 5. Identification of an Unknown Acid
6.
You are to identify an unknown that is one of the following compounds:
mp(oC) Name
17
acetic acid
103
3,3-dimethylglutaric
acid
13
acrylic acid
-23
propanoic acid
139-140 maleic acid
31-2 cyclohexane
carboxylic acid
-33.8 pentanoic acid
186-7 3-chloro, 1,2 benzene
dicarboxylic acid
-22.5 4-pentenoic acid
Formula
CH3COOH
HOOCCH2 C(CH3)2 CH2COOH
MW
60
160
H2C=CHCOOH
CH3CH2COOH
HOOCCH=CHCOOH
C6H11COOH
72
74
116
128
CH3CH2CH2CH2COOH
ClC6H4 (COOH)2
102
200
CH3=CHCH2CH2COOH
100
You titrate 0.374 g of the unknown with 0.100M NaOH. 37.03 mL of base is needed to
reach the end point. You now titrate 0.484 g of the unknown with 0.100M NaOH and 47.40
mL is required to reach the end point. What is the unknown's equivalent weight?
the equiv. weight =
Based solely on the calculated equivalent weight, list all compounds that may be the
unknown:
The unknown compound is a solid at room temperature (25oC). Based on this fact and
the calculated equivalent weight, identify the unknown.
Based on all the above tests and observations, the unknown most likely is:
(now try April’05, 4D; Dec.05, 4A; April ’06, 2C; Dec.06, 3 B-D; April’07, 2; Dec.07, 3)