Exp 4A: Conductivity Of Aqueous Solutions
• Purpose
– Study conductivity of a series of solutions to
determine the difference between strong electrolytes,
weak electrolytes and nonelectrolytes
– Use conductivity to distinguish between strong and
weak acids and strong and weak bases
– Use conductivity to study the effects of ion
concentration
• Conductivity of Solutions
The conductivity (or specific conductance) of an electrolyte solution is a measure of
its ability to conduct or allow the passage of electricity.
• Conductivity units
The SI unit of conductivity is siemens per meter (S/m)
• Conductivity vs Conductance
Consider a piece of wire.
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Electrical conductivity is a property of the material of the wire, and its value changes only with
temperature (it decreases linearly with temperature).
On the other hand conductance is the measure of the ease with which current can flow in the wire.
It depends on the physical parameters of the wire (length,area of cross section) as well as the
conductivity of the material of the wire.
conductance=conductivity x area of cross section / length
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APPLICATIONS and CONCEPT CONNECTIONS
Conductivity measurements are used routinely in many industrial and environmental
applications as a fast, inexpensive and reliable way of measuring the ionic content in
a solution. For example, the measurement of product conductivity is a typical way to
monitor and continuously trend the performance of water purification systems.
For water quality- Conductivity is linked directly to the total dissolved solids (T.D.S.).
High quality deionized water has a conductivity of about 5.5 μS/m,
Typical drinking water in the range of 5-50 mS/m
Sea water about 5 S/m (one million times higher than deionized water).
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Conductivity is traditionally determined by measuring the AC (alternating currect)
resistance of the solution between two electrodes.
Dilute solutions follow Kohlrausch's Laws of concentration dependence and additivity
of ionic contributions. Onsager gave a theoretical explanation of Kohlrausch's law by
extending the Debye–Hückel theory.
Exp 4A: Conductivity Of Aqueous Solutions
• Electrolytes
– Aqueous solutions of ionic compounds
• Ionic compounds dissolve and dissociate in water
NaCl(s)  Na+(aq) + Cl-(aq)
• Formation of positive and negative ions in solution
• Solution conducts electricity
• Strong electrolytes conduct electricity easily
– Strong electrolytes are completely dissociated (100%)
• Weak electrolytes conduct electricity poorly
– Weak electrolytes are only partially dissociates (<100%)
– Mostly undissociated = molecular form
CH3COOH(l)
H+(aq) + CH3COO-(aq)
• Nonelectrolytes
• Compounds that do not conduct electricity in solution
• Compounds that do not form ions in aqueous solution
Exp 4A: Conductivity Of Aqueous Solutions
• NaCl(s) + H2O(l)  Na+(aq) + Cl-(aq)
– 1 positive charge + 1 negative charge
• 2 ionic charges
• MgCl2(s) + H2O(l)  Mg2+(aq) + 2 Cl-(aq)
– 1 ion with +2 charge, 2 ions with –1 charge
• 4 ionic charges
• Fe(NO3)3+ H2O(l)  Fe3+(aq) + 3 NO3-(aq)
– 1 ion with +3 charge, 3 ions with –1 charge
• 6 ionic charges
• Conductivity depends on
– Concentration of ions
– Charge of ions
– (Size of ions (mobility in solution: large ions move more slowly))
Exp 4A: Conductivity Of Aqueous Solutions
Electrolytes conduct electricity
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Exp 4A: Conductivity Of Aqueous Solutions
• Strong electrolytes
– Conduct electricity easily
– Electrolyte (almost) completely dissociated in solution
NaCl(s) + H2O(l)  Na+(aq) + Cl-(aq)
• Weak electrolytes
– Poorly conducting solutions
– Electrolytes mostly in molecular form with few ions
NaH2PO4(s) + H2O(l)
Na+(aq) + H2PO4-(aq)
• Magnitude of conductance of a solution
– proportional to the number and type of ions in solution
• More ions (higher ion concentration): more conductivity
• More ionic charges: more conductivity
Exp 4A: Conductivity Of Aqueous Solutions
• Strong Acids
– HCl(g) + H2O(l)  H+(aq) + Cl-(aq)
– HNO3(l) + H2O(l)  H+(aq) + NO3-(aq)
• Strong Bases
– KOH(s) + H2O(l)  K+(aq) + OH-(aq)
• Weak Acids
– CH3COOH(l) + H2O(l)
H+(aq) + CH3COO-(aq)
• Weak Bases
– NH3(g) + H2O(l)
NH4+(aq) + OH-(aq)
Exp 4A: Conductivity Of Aqueous Solutions
• Strong and Weak electrolytes
Strong Electrolyte
Weak Electrolyte
Exp 4A: Conductivity Of Aqueous Solutions
• Mixtures of electrolytes
– Conductance shows additive effect if electrolytes do
not react with each other
• more ions, more charges, more conductivity
– If a chemical reaction occurs between the
electrolytes, the properties of the new substance(s)
determine conductivity
• typically reaction between a weak acid and a base
or a weak base and an acid
NH3(aq) + HCl(aq)  NH4+(aq) + Cl-(aq)
Exp 4A: Conductivity Of Aqueous Solutions
Dilution
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A certain amount of a solution added to an amount of solvent to
lower the concentration
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Add 10 mL of 1.0 M NaCl to 90 mL of H2O
Final volume = 10 mL + 90 mL = 100 mL
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Dilution = 1:10
Concentration = (10 mL x 1.0 M NaCl)/ 100 mL = 0.10 M NaCl
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The concentration changes. Does the total amount of NaCl particles
change?
Dilution formula:
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initial molarity (mol/L) x initial volume (L) =
final molarity (mol/L) x final volume (L) =
Mi x V i = M f x V f =
mol/L x L = mol
Exp 4A: Conductivity Of Aqueous Solutions
Prelab Question 4a
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How will you prepare 10 mL of 0.050 M HCl from 0.10 M HCl?
Answer
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Use dilution formula V1 x M1 = V2 x M2
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M1 = 0.10 M
M2 = 0.050 M
V2 = 10 mL
V1 = V2 x M2/M1 = 10 mL x 0.050 M/0.10M = 5.0 mL of 0.10 M HCl
Take 5.0 mL of 0.10 M hydrochloric acid in a 10.0-mL graduated
cylinder and dilute to 10 mL with dH2O
Exp 4A: Conductivity Of Aqueous Solutions
Prelab Question 5a
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How will you prepare 80 mL of 0.10 M CH3COOH from 6.0 M acetic acid?
Answer
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Use dilution formula V1 x M1 = V2 x M2
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M1 = 6.0 M
M2 = 0.10 M
V2 = 80 mL
V1 = V2 x M2/M1 = 80 mL x 0.10 M/6.0M = 1.3 mL of 6.0 M acetic acid
Take 1.3 mL of 6.0 M acetic acid in a 5.0-mL graduated cylinder or use a
5.0 mL pipet. Add ~ 50 mL dH2O to a 100-mL graduated cylinder, then
add the 1.3 mL of acetic acid. If you use a 5.0 mL cylinder for the acetic
acid, rinse it out with dH2O and add it to the 100-mL cylinder. Repeat this
process 2 more times. Add dH2O to a total volume of 80 mL.
Prelab Question 5b
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Same as 5a
Exp 4A: Conductivity Of Aqueous Solutions
Part 1: Compare conductance of
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20 mL dH2O, 20 mL tap water, 20 mL ethanol in 50 mL
beaker
Why is there a difference in conductance, if any, between
distilled water and tap water?
Part 2: Measure conductance of hydrochloric acid solutions
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20 mL 0.10 M HCl
0.050 M HCl: dilute 10 mL 0.10 M HCl + 10 mL dH2O
0.020 M HCl: dilute 10 mL 0.050 M HCl + 15 mL dH2O
How do you make dilutions?
 See prelab assignment 4!
Exp 4A: Conductivity Of Aqueous Solutions
Part 3: Measure conductivity in different solutions
Solution
Concentration
1
10 mL 0.10 M HNO3 + 10 mL dH2O
2
10 mL 0.10 M KOH + 10 mL dH2O
3
10 mL 0.10 M KCl + 10 mL dH2O
4
10 mL 0.10 M KNO3 + 10 mL dH2O
5
10 mL 0.10 M Ca(NO3)2 + 10 mL dH2O
6
10 mL 0.10 M NH3 + 10 mL dH2O
7
10 mL 0.10 M HC2H3O2 + 10 mL dH2O
8
10 mL 0.10 M HCl + 10 mL 0.10 M KNO3
Calculate new
concentrations
9
10 mL 0.10 M HNO3 + 10 mL 0.10 M KCl
Calculate new
concentrations
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10 mL 0.10 M HCl + 10 mL 0.10 M KOH
10 mL 0.10 M NH3 + 10 mL 0.10 M HC2H3O2
Calculate new
concentrations
Calculate new
concentrations
Conductance
Exp 4A: Conductivity Of Aqueous Solutions
Measure conductivity in different solutions
Solution
Concentration
1
10 mL 0.10 M HNO3 + 10 mL dH2O
0.05
2
10 mL 0.10 M KOH + 10 mL dH2O
0.05
3
10 mL 0.10 M KCl + 10 mL dH2O
0.05
4
10 mL 0.10 M KNO3 + 10 mL dH2O
0.05
5
10 mL 0.10 M Ca(NO3)2 + 10 mL dH2O
0.05
6
10 mL 0.10 M NH3 + 10 mL dH2O
7
10 mL 0.10 M HC2H3O2 + 10 mL dH2O
8
10 mL 0.10 M HCl + 10 mL 0.10 M KNO3
9
10 mL 0.10 M HNO3 + 10 mL 0.10 M KCl
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10 mL 0.10 M HCl + 10 mL 0.10 M KOH
10 mL 0.10 M NH3 + 10 mL 0.10 M HC2H3O2
Conductance
Exp 4A: Conductivity Of Aqueous Solutions
Next week:
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No Formal report.
Turn in the following, stapled in this order
• On paper: Type or write 1)a summary of the experiment and
2)your conclusions about the relationship between conductivity
and concentration
• Data and Calculations sheets for Exp 4A: Conductivity Of
Aqueous Solutions
• Answers to post-lab questions on the lab manual sheets
Prelab Assignment for Exp 4B: Ionic Reactions in Aqueous
Solutions
– Avoid getting definitions from Google or Wikipedia. Use a
textbook or lab manual or a science dictionary
– Read Prelab preparations and protocol
– Answer Prelab questions 1a-d, 2a-h, 3, 4