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ConductivityLaboratory - Conductance of Solutions Degree of...

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Conductance of Solutions Degree of Dissociation of Electrolytes Goal : To determine the limiting molar conductivities of strong electrolytes and--by inference--the limiting molar conductivity of a weak electrolyte. A second goal is to determine the dissociation constant of a weak acid as a function of concentration and the value of the thermodynamic equilibrium constant for the dissociation of the weak acid. Associated Sections in Text: 17.10 Theoretical Background CONDUCTIVITY The conductance of a solution can be readily measured using a conductivity meter. The value of conductance depends both on the properties of the meter and on the electrical conductivity, κ, of the solution. Electrical conductivity has units of S m -1 (Siemens per meter). Because the unit S = Ω -1 , the units are sometimes stated as mho per meter. LIMITING MOLAR CONDUCTIVITY Molar conductivity, Λ m , is the conductivity of a solution divided by its molar concentration, κ/c. Limiting molar conductivity is determined by extending a plot of molar conductivity to zero concentration. (1) m c o Λ = Λ 0 lim The limiting molar conductivity is easy to determine for strong electrolytes, because molar conductivity is a linear function of the square root of concentration (equation 2). (2) c o m Κ - Λ = Λ Derived by Kohlrausch in 1900, the y-intercept of this equation is the limiting molar conductivity. In addition, the experimentalist can assess the quality of data by how linear it is. As concentration is decreased the magnitude of intermolecular interactions between ions must decrease. At the limit of zero concentration, the identity of any counter-ion becomes unimportant. Consequently, the contribution of a given ion to the limiting molar conductivity is independent of the identity of the counter ion; i.e., it can be separately quantified.
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(3) - - + + + = Λ λ ν o It is virtually impossible to determine the limiting molar conductivity of weak electrolytes directly from experiment. Weak electrolytes are only completely dissociated at infinite dilution, at which concentration there are too few ions to be detected by a meter. As the concentration of a weak electrolyte increases, the degree of dissociation decreases, and so the number of ions doesn’t increase as rapidly as the concentration of the electrolyte. Consequently, the molar conductivity of weak electrolytes drops precipitously with concentration. A functional relationship between molar conductivity and concentration for weak electrolytes is presented by the Ostwald dilution law: (4) ( 29 2 1 1 o m a m o m m K c Λ Λ + Λ = Λ A more reliable way to determine the limiting molar conductivity of a weak electrolyte requires an indirect measurement and judicious use of equation (3). Concept Check
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ConductivityLaboratory - Conductance of Solutions Degree of...

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