Chemistry 223
Worksheet 2 Notes
Course ID Number ____________
Summer 2002
June 26, 2002
Dr. Nafshun
Score ____________
1.
List five species that are expected to be soluble in water. Polar (and possibly with
hydrogen bonding). CH3F, NH3, CH3OH, NO, O3 to name a few.
List five species that are expected to be soluble in CCl4. Non-polar, CH4, CO2, SF6,
C8H18, P4 to name a few.
List five species that are expected to be soluble in benzene (C6H6). Non polar—same as
CCl4.
List five species that are expected to be soluble in ethanol. Same as water—polar (and
possibly with hydrogen bonding).
2.
What is an azeotrope? Give an example. An azeotrope is a mixture of liquids that has a
constant boiling point and thus cannot be separated by distillation. Ethanol (boiling point
78.5 deg C) and water (boiling point 100 deg C) form a binary (two component)
azeotrope having a boiling point of 78.2 deg C and a composition that is 95.6% ethanol.
(Because the boiling point of the azeotrope is lower that the boiling points of either of the
two pure components, this is called a minimum boiling azeotrope.) Thus, a fractional
distillation of an ethanol-water mixture will produce a distillate that is 95% ethanol.
3.
What is meant by: a saturated solution, an unsaturated solution, a super-saturated
solution? Saturated: solute has been dissolved in a solvent so no more can be dissolved.
Unsaturated: solute has been dissolved in a solvent so more can be dissolved.
Supersaturated: more solute has been dissolved in a solvent than is soluble—a
metastable mixture... Solution that is more concentrated than a saturated solution is
known as super saturated solution. If a crystal of solute is added to this solution, the
excess of solute crystallizes.
4.
Draw an example of a soap molecule. What is meant by hydrophilic? What is meant by
hydrophobic? How does a soap work? What intermolecular forces are involved?
hydrocarbon end (hydrophobic—water fearing) /\/\/\/\/\/\/\/\/\/\/\/\_O polar end
(hydrophilic—water loving) The hydrocarbon end surrounds the grease (oil, blood, grass
stain—non polar molecule) and the hydrophilic end is attracted to the water. The non
polar interactions are dispersion forces and the water interactions are dipole-dipole.
5.
What forces are broken when ethylene glycol (CH2OHCH2OH) is dissolved in water?
What new forces are made? Hydrogen bonding, hydrogen bonding.
6.
Consider 1.0 m Na2CO3 (aq), 1.0 m KCl, 1.0 m sucrose (C12H22O11) (aq), 1.0 m AlCl3
(aq), and 1.0 m NaF (aq). Which solution has the greatest boiling point? The solution
with the greatest value for i. It is aluminum chloride with i = 4.
7.
A student places 15.00 grams of ammonium phosphate into 500 mL of water. Calculate
the molality of the solution. (Take dH2O to be 1.00 g/mL). What is i for ammonium
phosphate? i = 4. Calculate the freezing point of this solution.
ΔT = imk = (4)[(15.00 grams/149 g/mol)/0.500 kg](1.86 C/m) = 1.50 C
Tf = -1.50 C
Is an aqueous solution of sodium phosphate of similar concentration expected to freeze at
the same temperature as the ammonium phosphate solution? Explain. Molar mass is
different, so 15 grams will be a different number of moles.
8.
What mass of sucrose must be added to 250.0 g of water to increase the boiling point to
109.5 C?
ΔT = imk = (1)[(x grams/342 g/mol)/0.250 kg](0.52 C/m) = 9.5 C
9.
A student dissolves 17.50 g of an unknown protein in 200.0 g of water at 25.00 C. She
measures the osmotic pressure to be 0.0990 torr. Determine the molar mass of the
protein.
ΠV = nRT
n = ΠV / RT = [0.0990 torr (1 atm/760 torr)](0.200 L) / (0.0821 L atm/mol K)(298 K)
n = 1.1 x 10-6 moles
MWT = g/mol = 17.50 g / 1.1 x 10-6 moles = 1.6 x 107 g/mol
10.
List "the" three strong acids. Name them. Why are Ka values not provided for these
acids? Show the dissociation reactions for these acids. What is the definition of a strong
acid? Why are these strong? HCl, HNO3, H2SO4 are strong because they dissociate
100% due to the stability of the conjugate bases (Cl-, NO3-, and HSO4-). No Ka values
exist because no reactants exist—there is no equilibrium.
11.
Consider 1.0 M H2CO3 (aq). Write the two dissociation steps for H2CO3 (aq). What
species are present in 1.0 M H2CO3 (aq)?
H2CO3 (aq) ↔ HCO3- (aq) + H+ (aq)
and
HCO3- (aq) ↔ CO32- (aq) + H+ (aq)
12.
Draw the Lewis Structure for CH4. Is CH4 a strong acid? No. Is CH4 a strong base? No.
Draw the Lewis Structure for NH3. Is NH3 a strong acid? No. Is NH3 a strong base? No,
weak base.
13.
Is CH3CH2CH2CH2CH2CH2COOH a weak acid? Why? Yes, -COOH group.
14.
A student titrates 0.6831 grams of KHP (potassium hydrogen phthalate MW=204.2
g/mol) to the equivalence point with 35.42 mL of NaOH (aq). Determine the
concentration of the NaOH solution.
At the equivalence point,
moles KHP = moles NaOH.
(0.6831 g/204.2 g/mol) = (MNaOH)(0.03542 L)
MNaOH = 0.09445 M
15.
Determine the pH of 1.75 M CH2ClCOOH (aq) (see text for Ka value). Draw the Lewis
Structures for all species. ID the acid, base, conjugate acid, and conjugate base.
CH2ClCOOH (aq) + H2O (l) ↔ CH2ClCOO- (aq) + H3O+ (aq)
a
b
cb
ca
I
1.75
0
0
C
-x
+x
+x
E
1.75 – x
x
x
Ka = 1.4 x 10-3 = [CH2ClCOO- (aq)] [H3O+ (aq)] / [CH2ClCOOH (aq)] = x2 / 1.75 – x
(lose the x in the denominator)
x = [H+] = 4.95 x 10-2
pH = -log [H+] = -log (4.95 x 10-2) = 1.31
16.
Write the reaction for water and ammonia. Write the reaction for acetic acid and
potassium hydroxide. Write the reaction for nitric acid and ammonia. Write the reaction
for KHP and sodium hydroxide. For each of the previous reactions, identify the acid,
base, conjugate acid, and conjugate base.
17.
Which of the following has the LOWEST pH? Which of the following has the
HIGHEST pH? 1.50 M benzoic acid, 1.50 M chloroacetic acid, 1.75 M benzoic acid, or
1.75 M chloroacetic acid. Why?
Lowest pH will be the most concentrated of the strongest acid (greatest Ka, which means
most dissociation into conjugate base and protons (1.75 M chloroacetic acid).
Highest pH will be the least concentrated of the weakest acid (lowest Ka, which means
least dissociation into conjugate base and protons (1.50 M benzoic acid).
18.
A student titrates 25.00 mL of HCl (aq) with 35.22 mL of 0.09227 M NaOH (aq) to reach
the equivalence point. Determine the concentration of HCl (aq). What is the pH at the
equivalence point?
pH = 7 at the equivalence point.
At the equivalence point moles NaOH = moles HCl:
MV = MV
(0.09227 M)(35.22 mL) = (MHCl)(25.00 mL)
MHCl = 0.130 M
19.
What is the autoionization of water? Give the reaction.
Water automatically (all by itself) ionizes into hydroxide and proton (or hydronium ion if
you prefer to use H3O+ instead of H+).
H2O (l) + H2O (l) ↔ H3O+ (aq) + OH- (aq) Of course, Kw is only 1 x 10-14
20.
Determine the pH, pOH, [H+], and [OH-] of 0.1000 CH3COOH (aq). Determine the pH,
pOH, [H+], and [OH-] of 0.02500 KOH (aq). Determine the pH, pOH, [H+], and [OH-] of
0.02500 Ba(OH)2 (aq).
21.
Is 1.00 M C6H5COOH (aq) plus 1.00 M C6H5COONa (aq) a buffer system? Explain.
Yes, contains both a weak acid and a conjugate base. Is water a buffer system? No. Is
1.00 M CH3COOH (aq) plus 1.00 M C6H5COOH (aq) a buffer system? No, these are
both weak acids. Is 0.50 M C6H5COOH (aq) plus 1.00 M C6H5COONa (aq) a buffer
system? Yes, contains both a weak acid and a conjugate base.
22.
Determine the pH of a buffer system that is 1.00 M CH3COOH (aq) and 1.00 M
CH3COONa (aq).
Ka = 1.8 x 10-5 = [CH3COO- (aq)] [H+ (aq)] / [CH2ClCOOH (aq)] = (1.0)(x) / (1.0)
x = H+ (aq) = 1.8 x 10-5
pH = 4.74
23.
Determine the pH of a buffer system that is 3.00 M CH3COOH (aq) and 1.00 M
CH3COONa (aq). Is this pH what you expected compared to the pH in Question #22?
24.
Determine the pH of a buffer system that is 1.00 M CH3COOH (aq) and 3.00 M
CH3COONa (aq). Is this pH what you expected compared to the pH in Question #22?
25.
Which buffer system has the LOWEST pH? Why?
(A) 0.50 M C6H5COOH (aq) and 0.50 M C6H5COONa (aq)
(B) 1.00 M C6H5COOH (aq) and 0.50 M C6H5COONa (aq) More acid than base.
(C) 0.50 M C6H5COOH (aq) and 1.00 M C6H5COONa (aq)
26.
The pH of NaCl (aq) is: 7, less than 7, or greater than 7?
The pH of NH4NO3 (aq) is: 7, less than 7, or greater than 7? Acid and nitrate ion (nitrate
ion is a spectator ion)
The pH of C6H5COONa (aq) is: 7, less than 7, or greater than 7? Base and sodium ion
(sodium is a spectator ion).
Write two reactions: one for the dissociation of C6H5COONa and one responsible for
your pH selection.