PRE-LABORATORY QUESTIONS/ANSWERS
1.
Predict the geometry and color of a solution of CoCl2 dissolved in methanol (CH3OH) and 2-methyl2-propanol, (CH3)3COH. Write the chemical formula of the coordination complex in this solution.
2.
If the conversion of a tetrahedral cobalt complex to an octahedral complex is exothermic, ΔH= -31
kJ mol-1, what will be the effect of increasing temperature on the Keq and on the color of the
solution?
3.
To determine the extinction coefficient ε of the tetrahedral complex CoBr2P2, a student dissolves
0.26 g of CoBr2·6H2O in 50.00 mL of 2-propanol to make a stock solution. He makes up four
solutions from this stock as follows:
Solution
Volume of stock CoBr2
solution, mL
Volume of 2propanol, mL
Absorbance at
670 nm
1
0.50
9.50
0.303
2
1.00
9.00
0.577
3
1.50
8.50
0.855
4
2.00
8.00
1.149
Calculate the concentration of CoBr2P2 in each solution. Plot A670 against concentration.
Determine ε.
4.
The stock solution in question 3 is used to explore the equilibrium
CoBr2P2 + 3 M D CoBr2PM3 + P
or, more concisely,
[Co(tet)] + 3 M D [Co(oct)] + P
In a sample containing 1.00 mL stock CoBr2 solution, 2.00 mL methanol and 7.00 mL 2-propanol,
the absorbance at 670 nm is 0.181 at 20 oC. The density of methanol is 0.791 g/mL and that of 2propanol is 0.785 g/ml.
a.
b.
c.
d.
e.
f.
Write the equation for Keq.
Calculate the total concentration of Co, [Co(tet)] +[Co(oct)].
Calculate [Cotet].
Calculate [Co(oct)].
Calculate the concentrations of methanol and 2-propanol.
Calculate Keq.
Answers:
1.
Predict the geometry and color of a solution of CoCl2 dissolved in methanol (CH3OH)and 2-methyl2-propanol, (CH3)3COH. Write the chemical formula of the coordination complex in this solution.
When CoCl2 dissolved in CH3OH, it forms [Co(CH3OH)6 ],2+ (CH3)3COH is much bulkier than
CH3OH and surely would not be able to fit into an octahedral complex. CoCl2((CH3)3COH)4 or
(Co((CH3)3COH)42+will be tetrahedral and blue.
2.
If the conversion of a tetrahedral cobalt complex to an octahedral complex is exothermic, ΔH= -31
kJ mol-1, what will be the effect of increasing temperature on the Keq and on the color of the
solution?
Based on the Van’t Hoff equation, a negative ΔH gives a positive slope of LnKsp v.s. 1/T. Keq will
decrease as increasing temperature and the solution will become more blue (more tetrahedral
complexes)
To determine the extinction coefficient ε of the tetrahedral complex CoBr2P2, a student dissolves
0.26 g of CoBr2·6H2O in 50.00 mL of 2-propanol to make a stock solution. He makes up four
solutions from this stock as follows:
Solution
Volume of stock CoBr2
solution, mL
Volume of 2propanol, mL
Absorbance at
670 nm
1
0.50
9.50
0.303
2
1.00
9.00
0.577
3
1.50
8.50
0.855
4
2.00
8.00
1.149
Calculate the concentration of CoBr2P2 in each solution. Plot A670 against concentration.
Determine ε.
standard curve
stock [CoP4]=
(0.26 g)/326.7 g/mol)(0.0500 L) =
0.016 M
1.2
y = 360.96x
R2 = 0.9992
1
0.8
A670
3.
0.6
0.4
sample [CoP4]=
0.2
0
0
0.001
0.002
0.003
(0.016 M)(Volsample (mL))/10.00 mL
[Co complex]
ε = 3.6 x 102 M-1
4.
The stock solution in question 3 is used to explore the equilibrium
CoBr2P2 + 3 M D CoBr2PM3 + P
or, more concisely,
[Co(tet)] + 3 M D [Co(oct)] + P
In a sample containing 1.00 mL stock CoBr2 solution, 2.00 mL methanol and 7.00 mL 2-propanol,
the absorbance at 670 nm is 0.181 at 20 oC. The density of methanol is 0.791 g/mL and that of 2propanol is 0.785 g/ml.
a.
Write the equation for Keq.
!
$
#"Co(oct) &%[ P ]
K =
eq !
$ 3
#"Co(tet) &%[ M ]
b.
Calculate the total concentration of Co, [Co(tet)] +[Co(oct)].
sample [Cototal]=(0.016 M)(1.00 mL)/(10.00 mL) = 0.0016 M
c.
Calculate [Cotet].
[Co(tet)]= A/ε = 0.181/(3.6 x 102 M-1)= 0.00050 M
d.
Calculate [Co(oct)].
[Co(oct)]= [Cototal] - [Cotet ]= 0.0016 – 0.00050 = 0.0011 M
e.
Calculate the concentrations of methanol and 2-propanol.
[methanol] = (2.00mL)(0.791g/mL)(1 mol/32.04 g)(1/.0100 L) = 4.94M
[2-propano] = (8.00mL)(0.785g/mL)(1 mol/60.09 g)(1/0.0100 L) = 10.5M
f.
Calculate Keq.
K eq =
!
(0.0011)(10.5) = 0.19
3
(0.00050)(4.94 )