Answers to Study Guide for CHEM 102 Test 4, Fall 2007
- Define the terms below:
- Bronsted-Lowry acid - a proton donor
- Bronsted-Lowry base - a proton acceptor
- Lewis acid - an electron pair acceptor
- Lewis base - an electron pair donor
- amphoteric - having both basic and acidic properties
- pH the -log[H+]
- pOH - the -log[OH-]
- What is the pH of an aqueous solution that is 0.00897 M in hydrogen ion (H+(aq)) concentration? What is the pOH?
pH = -log [H+] = -log(0.00897) = -(-2.05) = 2.05
pH + pOH = 14 so pOH = 14 -pH = 14 -2.05 =11.95
- What is the hydroxide ion concentration ( [OH-] ) of an aqueous solution that has a pH of 13.31? What is the hydrogen ion concentration ( [H+] )?
pH = -log[H+] so [H+] = 10-pH = 10-13.31
= 4.90 x 10-14
pOH = 14 - pH = 14 - 13.31 = 0.69
pOH = -log[OH-] so [OH-] = 10-pOH = 10-0.69 = 0.204 M
- Put the acids in each part below in order from strongest to weakest.)
- HClO, HClO4, HClO2, HClO3
HClO4> HClO3> HClO2> HClO
- H3SbO4, H3PO4, H3BiO4, H3AsO4
H3PO4>H3AsO4>H3SbO4>H3BiO4
- Find the pOH and pH of the solutions below:
- a solution where [H+] = 3.86 x 10-16
pH = 15.41, pOH = -1.41
- a solution where [OH-] = 6.22 x 10-6
pOH = 5.21, pH = 8.79
- a solution where [OH-] = [H+]
pH = 7, pOH = 7
- a solution where the hydroxide ion concentration is equal to double the hydrogen ion concentration
The product of the concentrations must equal 1 x 10-14
(2x)x = 1 x 10-14 = 2x2
x2 = 5 x 10-15
x = (5 x 10-15)1/2 = 7.07 x 10-8 M = [H+]
pH = 7.15, pOH = 6.85
- a solution where [OH-] = 1.06 x 10-12
pOH = 11.97, pH = 2.03
- What is the pH of a solution that is 4.850 M in NaNO2 and 0.750 M in HNO2? The KA for HNO2 is 4.5 x 10-4.
HNO2(aq) º H+(aq) + NO2-(aq)
Ka[HNO2]}/[NO2-] = {(4.5 x 10-4)(0.750)}/(4.850) = [H+]
[H+] = 6.96 x 10-5
pH = -log[H+] = - log(6.96 x 10-5) = 4.16
- What is the pH of a buffer solution that is 2.00 M in sodium benzoate (NaOOCC6H5) and 0.100 M in benzoic acid (HOOCC6H5)? The KA of benzoic acid is 6.3 x 10-5.
HOOCC6H5(aq) <-----------> H+(aq) + -OOCC6H(aq)
Ka = {[H+][-OOCC6H5]}/[HOOCC6H5]
{Ka[HOOCC6H5]}/[-OOCC6H5] = [H+]
{(6.3 x 10-5)(0.100)}/(2.00) = 3.15 x 10-6 = [H+]
pH = -log[H+] = -log(3.15 x 10-6) = 5.50
- Using the buffer solution in the problem above, what is the pH if 15.00 mL of 2.50 M HCl(aq) is added to 75.00 mL of the buffer solution? What would be the pH if 25.00 mL of 1.45 M NaOH was added to 85.00 mL of the buffer solution in problem the problem above?
Using the acid solution:
Dilution problems:
[HOOCC6H5] = {(0.100 M)(75.00 mL)}/(90.00 mL) = 0.0833 M
[-OOCC6H5] = {(2.00 M)(75.00 mL)}/(90.00 mL) = 1.67 M
Added acid:[H+] = {(2.50 M)(15.00 mL)}/(90.00 mL) = 0.417 M
The added acid will decrease the concentration of the benzoate ion, -OOCC6H5,
new [-OOCC6H5] = 1.67 - 0.417 = 1.253 M
The added acid will increase the benzoic acid, HOOCC6H5,
new [HOOCC6H5] = 0.0833 + 0.417 = 0.500 M
Rearranging Ka = {[H+][-OOCC6H5]}/[HOOCC6H5]
to solve for [H+] yields [H+] = {Ka[HOOCC6H5]}/[-OOCC6H5]
[H+] = {(6.3 x 10-5)(0.500)}/(1.253) = 2.61 x 10-5
pH = -log[H+] = -log(2.61 x 10-5) = 4.58
For added base:
Dilution problems:
[-OOCC6H5] = {(2.00 M)(85.00 mL)}/(110.00 mL) = 1.545 M
[HOOCC6H5] = {(0.100 M)(85.00 mL)}/(110.00 mL) = 0.0773 M
added base:
[OH-] = {(1.45 M)(25.00 mL)}/(110.00 mL) = 0.330 M
The added base will decrease the acid concentration,
0.0773 M - 0.330 M = -0.2527 M
The negative number indicates that this buffer does not have enough capacity to handle this amount of base, leaving an excess
of 0.2527 M in hydroxide. Therefore the pOH of the solution is:
pOH = -log[OH-] = -log(0.2527) = 5.97
pH = 14 - pOH = 14 - 5.97 = 8.03
- What is the percent ionization for the following solutions of acids?
- 2.50 M HC2H3O2, KA = 1.8 x 10-5
HC2H3O2(aq) º H+(aq)
+ C2H3O2-(aq)
KA = {[H+][C2H3O2-]}/[HC2H3O2]
= x2/[HC2H3O2]
KA[HC2H3O2]
= x2 = (1.8 x 10-5)(2.50) = 4.5 x 10-5
x = (4.5 x 10-5)1/2 = 6.71 x 10-3 = [H+] = [C2H3O2-]
% ionization = {(6.71 x 10-3)/2.50} x 100 = 0.268%
quadratic not needed
- 0.00500 M HCOOH, KA = 1.7 x 10-4
18.4%, quadratic needed
- 0.1500 M C6H5OH, KA = 1.3 x 10-10
0.00294%, quadratic not needed
- 0.000515 M HF(aq), KA = 7.1 x 10-4
completely ionized!! might be small difference with quadratic
- 0.00180 M HC9H7O4, KA = 3.0 x 10-4
40.8%, quadratic needed
- What is the sulfate concentration in a 14.00 M H2SO4(aq) solution? Ka1 = extremely large, ~100 % dissociation, KA2 = 1.3 x 10-2.
H2SO4(aq) 6 H+(aq)
+ HSO4-(aq),
[H+] = [HSO4-] = 14.00 M
HSO4-(aq) º H+(aq) + SO42-(aq)
KA2 = {[H+][SO42-]}/[HSO4-] but [H+]
= [HSO4-], since second dissociation provides an insignificant amount of H+
compared to first dissociation thus KA2 = [SO42-] = 1.3 x 10-2.
- What is the arsenate concentration (AsO43-) in 3.00 M arsenic acid (H3AsO4)? KA1 = 6.0 x 10-3, KA2 = 1.05 x 10-7, KA3 = 3.0 x 10-12
H3AsO4(aq) º H+(aq) + H2AsO4-(aq)
% ionization calculation:
KA1 = {[H+][H2SO4-]}/[H3AsO4]
= x2/[H3AsO4]
KA1[H3AsO4] = x2
(6.0 x 10-3)(3.00) = 1.80 x 10-2 = x2
(1.8 x 10-2)1/2 = x = 0.134
% ionization = {0.134/3.00} x 100 = 4.47%, quadratic not needed, thus [H+] = [H2AsO4-] = 0.134 M
H2AsO4-(aq)º H+(aq)
+ HAsO42-(aq)
KA2 = {[H+][HAsO42-]}/[H2AsO4-]
but [H+] = [H2AsO4-] so
KA2 = [HAsO42-]
HAsO42-(aq) º
H+(aq) + AsO43-(aq)
KA3 = {[H+][AsO43-]}/[HAsO42-]
KA3[HAsO42-]/[H+] = [AsO43-] but [H+] = 0.134 and [HAsO42-]
= KA2
so KA3KA2/[H+] = [AsO43-]
{(3.0 x 10-12)(1.05 x 10-7)}/(0.134) = 2.35 x 10-18 M = [AsO43-]
- What is the pH of a solution that is
HYDROLYSIS!!
- 6.50 M in NaC2H3O2, HC2H3O2 KA = 1.8 x 10-5
C2H3O2-(aq)
+ H2O º HC2H3O2(aq)
+ OH-(aq)
KB = (1.0 x 10-14)/KA = (1.0 x 10-14)/(1.8 x 10-5)
KB = 5.56 x 10-10
KB = {[H2C2H3O2][OH-]}/[C2H3O2-]
= x2/[C2H3O2-]
KB[C2H3O2-]
= x2 = (5.56 x 10-10)(6.50)
x2 = 3.614 x 10-9<
x = (3.614 x 10-9)1/2 = 6.01 x 10-5 = [OH-]
pOH = -log[OH-] = -log(6.01 x 10-5) = 4.22
pH = 14 -pOH = 14 -4.22 = 9.78
- 2.60 M in NaCN, HCN KA = 2.1 x 10-9
same method as a, pH = 11.55
- 4.50 M in NH4Cl, NH3 KB = 1.8 x 10-5
NH4+(aq)
º H+(aq) + NH3(aq)
KA = Kw/KB = (1.0 x 10-14)/(1.8 x 10-5) = 5.56 x 10-10
KA = {[H+][NH3]}/[NH4+] = x2/[NH4+]
KA[NH4+] = x2 = (5.56 x 10-10)(4.50) = 2.502 x 10-9
x = (2.502 x 10-9)1/2 = 5.00 x 10-5 = [H+]
pH = -log[H+]
= -log(5.00 x 10-5) = 4.30
- 2.34 M in NaIO3, HIO3 KA = 1.58 x 10-1
same method as a, pH = 7.59
- 2.95 M in NaN3, HN3 KA = 1.9 x 10-5
same method as a, pH = 9.60
- What is the pH of a buffer that is 3.93 M in NaCN and 0.951 M in HCN? If 10.00 mL of 0.1500 M HCl is added to 100.00 mL of this buffer what is the pH? If 15.00 mL of 0.6000 M NaOH is added to 95.00 mL of this buffer what is the pH?
HCN(aq)º H+(aq)
+ CN-(aq) KA = 2.1 x 10-9
KA = {[H+][CN-]}/[HCN] = 2.1 x 10-9
KA[[HCN]/[CN-] = [H+] = {(2.1 x 10-9)(0.951)/(3.93)
[H+] = 5.08 x 10-10
pH = -log[H+] = -log(5.08 x 10-10) = 9.29
Added acid problem:
Dilutions:
[CN-] = {(3.93 M)(100.00 mL)}/(110.00 mL) = 3.57 M
[HCN] = {(0.951 M)(100.00 mL)}/(110.00 mL) = 0.865 M
added acid:
[H+] = {(0.1500 M)(10.00 mL)}/(110.00 mL) = 0.0136 M
Added acid will decrease [CN-]
new [CN-] = 3.57 - 0.0136 = 3.5564 M
Added acid will increase [HCN]
new [HCN] = 0.865 + 0.0136 = 0.8786 M
KA[HCN]/[CN-] = [H+] = {(2.1 x 10-9)(0.8786)}/(3.5564)
[H+] = 5.19 x 10-10, pH = 9.28
Added base problem:
Dilutions:
[CN-] = {(3.93 M)(95.00 mL)}/(110.00 mL) = 3.39 M
[HCN] = {(0.951 M)(95.00 mL)}/(110.00 mL) = 0.821 M
added base:
[OH-] = {(0.6000 M)(15.00 mL)}/(110.00 mL) = 0.0818 M
Added base will decrease [HCN] and increase [CN-]
new [CN-] = 3.39 + 0.0818 = 3.4718 M
new [HCN] = 0.821 -0.0818 = 0.7392 M
A[HCN]/[CN-] = {(2.1 x 10-9)(0.7392)}/(3.4718) = [H+]
[H+] = 4.47 x 10-10, pH = 9.35
HCN(aq)º H+(aq) + CN-(aq)
- Design buffers with the at pH = 5.00 with the following
- a system 1.00 M in HC2H3O2, KA = 1.8 x 10-5
pH 5 means [H+] = 1.00 x 10-5
HC2H3O2(aq) º H+(aq)
+ C2H3O2-(aq)
KA = {[H+][C2H3O2-]}/[HC2H3O2]
KA[HC2H3O2]/[H+] =
{(1.8 x 10-5)(1.00)}/(1.00 x 10-5)=[C2H3O2-] = 1.80 M
- a system 0.750 M in NaCN, HCN KA = 2.1 x 10-9
[HCN] = 1.575 x 10-4 M
- a system 1.00 M in NH3, KB = 1.8 x 10-5
[NH4+] = 17985.6 M, a ridiculous answer, since the molarity of water in itself is the maximum
at 55.55 M. You cannot make a buffer at this pH with these concentrations with this system .
- a system 1.00 M in NH4Cl, NH3 KB = 1.8 x 10-5
[NH3] = 5.56 x 10-5 M. Although possible to make this buffer, it actually has almost no capacity.
- a system 1.00 M in NaC2H3O2, HC2H3O2 KA = 1.8 x 10-5
[HC2H3O2] = 0.556 M
- What are the equilibrium concentrations of the reactants and products below if a solution was made that was initially 1.00 M in Cl3CCOOH(aq), 0.500 M in H+(aq) and
0.325 M in Cl3CCOO-(aq)? KA = 1.29 x 10-1
Cl3CCOOH(aq) º H+(aq)
+ Cl3CCOO-(aq)
KA = {[H+][Cl3CCOO-]}/[Cl3COOH]
>
KA = {[0.500 + x][0.325 + x]}/[1.00 - x]
KA[1.00 - x] = [0.500 + x][0.325 + x]
0.129 - 0.129x = x2 + 0.825x + 0.1625
0 = x2 + 0.954x + 0.0335
x = {-b + (b2 - 4ac)1/2}/2a
x = {-0.954 + [(0.954)2 -4(1)(0.0335)]1/2}/2(1)
x = {-0.954 + (0.910116 - 0.134)1/2}/2
x = {-0.954 + (0.776116)1/2}/2
x = {-0.954 + 0.881}/2
x = -0.0365 or -0.9175
The only root that makes sense is the first since the second gives negative concentrations for [H+] and [Cl3COO-]
Thus the equilibrium concentrations are:
[Cl3COOH] = 1.00 - (-0.0365) = 1.0365 = 1.04 M
[Cl3COO-] = 0.325 + (-0.0365) = 0.2885 = 0.288 M
[H+] = 0.500 + (-0.0365) = 0.4635 = 0.464 M
- What is the molar solubility of PbSO4 if the Ksp for PbSO4 is
1.3 x 10-8?
PbSO4(s) º Pb2+(aq)
+ SO42-(aq), Ksp = [Pb2+][SO42-]
Since one mole yields 1 mole of Pb2+
and 1 mole of SO42- Ksp = x2 therefore, x, the molar
solubility is equal to
(Ksp)1/2 = (1.3 x 10-8)1/2 = 1.14 x 10-4 moles/L
- What is Ksp of CaF2 if the molar solubility of CaF2 is 2.136 x 10-4 M?
CaF2(s) º Ca2+(aq) + 2 F-(aq), Ksp
= [Ca2+][F-]2
therefore Ksp =(x)(2x)2 = 4x3 but x is equal to
the molar solubility so Ksp = 4(2.136 x 10-4)3
= 3.898 x 10-11
- If the molar solubility of Fe(OH)2 is 5.85 x 10-6, what is Ksp for Fe(OH)2?
Fe(OH)2(s) º Fe2+(aq) + 2 OH-(aq)
Ksp = [Fe2+][OH-]2
Using the same arguments as in the previsou problems, 5.85 x 10-6 moles of Fe2+ will be
present, but twice as many moles (1.17 x 10-5) of OH-will
be present.
Ksp = (5.85 x 10-6)(1.17 x 10-5)2 = 8.01 x 10-16
- If the Ksp for Mn(OH)2 is 2.00 x 10-13, what is the molar solubility of
Mn(OH)2?
Mn(OH)2(s) º Mn2+(aq) + 2 OH-(aq)
For every Mn(OH)2 that dissociates one Mn2+ is formed but two OH- s are formed thus the
molar solubility may be calculated by using x for the Mn2+ concentration and 2x for the OH- concentration.
The quantity "x" will be equal to the molar solubility.
Ksp = x(2x)2 = 4x3
Ksp/4 = x3
(Ksp/4)1/3 = x
(2.00 x 10-13/4)1/3 = x
(5.00 x 10-14)1/3 = x = 3.68 x 10-5 mol/L
- What is the molar solubility of AgCl in 1.00 M in NaCN?
KspAgCl = 1.6 x 10-10, KfAg(CN)2- = 1.0 x 1021
(s) º Ag+(aq) + Cl-(aq)
Ag+(aq) + 2 CN-(aq) º Ag(CN)2-(aq)
Overall rxn: AgCl(s) + 2 CN-(aq)º
Ag(CN)2-(aq) + Cl-(aq)
K = KspKf = {[Ag(CN)2-][Cl-]}/[CN-]2
K = (1.6 x 10-10)(1.0 x 1021) = 1.6 x 1011
1.6 x 1011 =(s)(s)/(1.0-2s)2
1.6 x 1011 = s2/(1.0-2s)2
(1.6 x 1011)1/2 = 4.0 x 105 = s/(1.0 -2s)
4.0 x 105 - (8.0 x 105)s = s
4.0 x 105 = (8.0 x 105)s + s
s = 0.500 moles/L for molar solubility
- What is the molar solubility of Zn(OH)2? What is its molar solubility in a solution buffered at pH 14? At pH 5? The solubility product for Zn(OH)2 is 2.1 x 10-14.
Zn(OH)2(s) º Zn2+(aq)
+ 2 OH-(aq)
Ksp = [Zn2+][OH-]2
Ksp = x(2x)2 = 4x3
Ksp/4 = 5.25 x 10-15 = x3
x = (5.25 x 10-15)1/3 = 1.74 x 10-5 M = molar solubility
In a pH 14 buffer pOH = 14-14 = 0 so [OH-] = 100 = 1.00
so Ksp/[OH-]2 = [Zn2+] = (2.1 x 10-14)/(1.00)2 = 2.1 x 10-14 which is the molar solubility
In pH 5 buffer, pOH = 14 -5 = 9, [OH-] = 1.00 x 10-9
[Zn2+] = Ksp/[OH-]2 = (2.1 x 10-14)/(1.00 x 10-9)2
[Zn2+] = 2.1 x 104 M (larger than maximum molarity, very soluble)
- Define:
- Ksp - the solubility product equal to the reaction quotient of the equilibrium expression of a salt of limited aqueous solubility
- Kf - formation constant for complex ions
- buffer - a solution that resist pH change
- buffer capacity - refers to how much added acid or base a buffer can absorb before coming non-functional
- Kd dissociation constant for a complex ion, the inverse of Kf, i.e. 1/Kf
- common ion effect - refers to the shift in equilibrium that occurs when an ion common to the equilibrium is added (i.e. the effect of
LeChatelier's principle)
- pKa = -logKa, which during a pH titration of an acid is equal to the pH at the half-equivalence point
- pKb = -lofKb
- titration - a volumetric method for determining concentration of a material in solution
- molar soulubility - how many moles of a material can be dissolved in 1.00 L of water
- complex ion - an ion formed by the association of another species with an metal ion
(eg.
Ag+(aq) + 2 NH3(aq) <-------> Ag(NH3)2+(aq))
- What is the solubility of AgCl in 5.00 M NH3?
AgCl(s) K - s
| <---------> |
Ag+(aq) s
| + |
Cl-(aq) s
| Ksp = 1.8 x 10-10 |
Ag+(aq) s | + |
2 NH3(aq) 5.00 - 2s | <--------> |
Ag(NH3)2+(aq) s |
Kf = 1.7 x 107 |
KC = KspKf = (1.8 x 10-10)(1.7 x 107) = 3.06 x 10-3
KC = [Ag+][Cl-][Ag(NH3)2+]/[Ag+][NH3]2
KC = [Cl-][Ag(NH3)2+]/[NH3]2
KC = s2/(5.00 - 2s)2
(KC)1/2 = (s2/(5.00 - 2s)2)1/2 = s/(5.00 - 2s) = (3.06 x 10-3)1/2 = 5.53 x 10-2
s/(5.00 - 2s) = 5.53 x 10-2
s = (5.00 -2s)(5.53 x 10-2
s = 0.2765 - 0.1106s
s + 0.1106 s = 0.2765
1.1106s = 0.2765
s = 0.2765/1.1106 = 0.249 M
- Explain how the pKA of an acid may be determined with a pH titration
In general a weak acid may be reperesented by the equation:
HA(aq) <--------> H+(aq) + A-(aq)
thus KA = [H+][A-]/[HA]
But at the half-equivalence point, half of the the HA has been consumed, and the concentrations of [HA] and [A-] will be equal
thus at the half-equivalence point KA = [H+]
so at the half equivlance point -logKA = -log[H+]
and pKA = pH
- Are the solutions below acidic or basic?
- 3.00 M Na+F- - contains the conjugate base of the weak acid HF, solution will be basic
- 3.00 M Na+C2H3O2- - contains the conjugate base of the weak acid
HC2H3O2, solution will be basic
- 3.00 M NH4+Cl- - contains the conjugate acid of the weak base NH3, solution will be acidic
- 3.00 M CH3CH2NH3+Cl- - contains the conjugate acid of the weak base
CH3CH2NH2, solution will be acidic.
- What is the molar solubility of AgCl in 1.00 M in NaCN?
KspAgCl = 1.6 x 10-10, KfAg(CN)2- = 1.0 x 1021
(s) º Ag+(aq) + Cl-(aq)
Ag+(aq) + 2 CN-(aq) º Ag(CN)2-(aq)
Overall rxn: AgCl(s) + 2 CN-(aq)º
Ag(CN)2-(aq) + Cl-(aq)
K = KspKf = {[Ag(CN)2-][Cl-]}/[CN-]2
K = (1.6 x 10-10)(1.0 x 1021) = 1.6 x 1011
1.6 x 1011 =(s)(s)/(1.0-2s)2
1.6 x 1011 = s2/(1.0-2s)2
(1.6 x 1011)1/2 = 4.0 x 105 = s/(1.0 -2s)
4.0 x 105 - (8.0 x 105)s = s
4.0 x 105 = (8.0 x 105)s + s
s = 0.500 moles/L for molar solubility
- What is the molar solubility of Zn(OH)2? What is its molar solubility in a solution buffered at pH 14? At pH 5? The solubility product for Zn(OH)2 is 2.1 x 10-14.
Zn(OH)2(s) º Zn2+(aq)
+ 2 OH-(aq)
Ksp = [Zn2+][OH-]2
Ksp = x(2x)2 = 4x3
Ksp/4 = 5.25 x 10-15 = x3
x = (5.25 x 10-15)1/3 = 1.74 x 10-5 M = molar solubility
In a pH 14 buffer pOH = 14-14 = 0 so [OH-] = 100 = 1.00
so Ksp/[OH-]2 = [Zn2+] = (2.1 x 10-14)/(1.00)2 = 2.1 x 10-14 which is the molar solubility
In pH 5 buffer, pOH = 14 -5 = 9, [OH-] = 1.00 x 10-9
[Zn2+] = Ksp/[OH-]2 = (2.1 x 10-14)/(1.00 x 10-9)2
[Zn2+] = 2.1 x 104 M (larger than maximum molarity, very soluble)