Molar conductance at infinite dilution

The salts had a high electrical conductivity, and it was claimed that the values of the molar conductances at infinite dilution showed the formation of a binary and ternary electrolyte respectively. 

For strong electrolytes the molar conductivity increases as the dilution is increased, but it appears to approach a limiting value known as the molar conductivity at infinite dilution. The quantity A00 can be determined by graphical extrapolation for dilute solutions of strong electrolytes. For weak electrolytes the extrapolation method cannot be used for the determination of Ax but it may be calculated from the molar conductivities at infinite dilution of the respective ions, use being made of the Law of Independent Migration of Ions . At infinite dilution the ions are independent of each other, and each contributes its part of the total conductivity, thus ... 

The value for k will normally decrease as the concentration of the solution decreases but the value for A will increase because of the increased dissociation of molecules in dilute solutions. A value for the molar conductance at infinite dilution (A,)) can be determined by plotting the calculated values for A against the molar concentration of the solution used and determining the plateau value for A. From such investigations it is possible to determine the ionic mobilities of ions (Table 4.3) and calculate the molar conductance of an... 

The conductance of an electrolyte solution characterizes the easiness of electric conduction its unit is reciprocal ohm, = siemens = S = A/V. The electric conductivity is proportional to the cross-section area and inversely proportional to the length of the conductor. The unit of conductivity is S/m. The conductivity of an electrol3de solution depends on the concentration of the ions. Molar conductivity, denoted as X, is when the concentration of the hypothetical ideal solution is 1 M = 1000 mol/m. Hence, the unit of molar conductivity is either Sm M , or using SI units, Sm mol . For nonideal solutions, X depends on concentration, and the value of X at infinite dilution is denoted by subscript "0" (such as >,+ 0, and X for cation and anion molar conductivity). The conductivity is a directly measurable property. The molar conductivity at infinite dilution may be related to the mobility as follows ... 

In conductimetric studies of triple ion formation, it is often assumed that M2A+ and MA2 have the same formation constants (Kt= Kt+= Kt ) and the same molar conductivities at infinite dilutions (2 = = 2 ). If we denote by a and a, the... 

A0 Molar conductivity at infinite dilution SmJ/kmol M- N- T3A2... 

The ionic conductivity of the selenite ion in water was determined by measuring the molar conductivities of a series of aqueous solutions of lithium selenite with concentrations below 2 x 10" M. The molar conductivity at infinite dilution was found by extrapolation to be (172.0 2.9) ohm -cm mor. With the ionic conductivity of Li equal to 33.4 ohm -cm -mor, the ionic conductivity of the selenite ion becomes 105.2 ohm -cm -moT. This value was then combined with the ionic conductivity of the metal ion to find the molar conductivity of the dissolved selenite. 

Calculate the limiting molar conductivities at infinite dilution, A", for the following weak acids at 25°C. 

Since AgBr(s) is a highly insoluble salt, then a saturated solution of AgBr(aq) will be a very dilute solution of Ag" "(aq) and Br (aq), and so the molar conductivity. A, for the samrated solution can be taken to be the hmiting value of the molar conductivity at infinite dilution, i.e. A. ... 

We can extrapolate the curves back to zero concentration and obtain a quantity known as Aq, the molar conductivity at infinite dilution, or zero concentration. With weak electrolytes this extrapolation may be unreliable, and an indirect method, explained on p. 285, is usually employed. It is convenient to denote the ratio of A at any concentration to Ao by the symbol a ... 

Table 6.2 Molar Conductivities at Infinite Dilution for Various Sodium and Potassium Salts in Aqueous Solution at 25 C...

An important relationship between molar conductivity and viscosity was discovered in 1906 by the Russian-German chemist Paul Walden (1863-1957). In the course of a study of the conductivity of tetramethylammoniura iodide in various solvents, Walden noticed that the product of the molar conductivity at infinite dilution and the viscosity rj of the solvent was approximately constant ... 

Equation (31.27) expresses the molar conductivity as the sum of independent contributions f rom each kind of ion present this is Kohlrausch s law it is strictly correct only if the electrolytic solution is infinitely dilute, c = 0. This is not surprising, since the electrically charged ions should exert a mutual influence on each other, especially if they are present in appreciable concentration. Thus, if A°° is the molar conductivity at infinite dilution, then the expression for Kohlrausch s law is... 

Here A is the molar conductance at concentration c, y is the mean ionic activity coefficient, A° is the molar conductance at infinite dilution, and iS is a function defined by Shedlovsky. ... 

A° being the molar conductance at infinite dilution, and the viscosity rj is in poise. 

According to the equations above, the molar conductivity A should be independent of the concentration. However, this is only true in the limit of infinite dilution. In order to ensure the validity of the equation, we replace A by A , which is the molar conductivity at infinitely low concentrations (molar conductivity at infinite dilution or limiting molar conductivity)-. 

Several matter dynamical constants can be determined by measuring conductivities, such as molar conductivity at infinite dilution or dissociation constants of weak electrolytes (compare Sect. 21.5). Conductivity measurements are also useful for kinetic investigations (see Experiment 16.9). 

Plotting A over Vc the molar conductivity at infinite dilution A can be determined. 

Some authors call this the Einstein equation, preferring to save the term Nernst-Einstein equation to describe the link between the molar conductivity at infinite dilution and the diffusion coefficients of an electrolyte... 

Table 4.2 - Molar conductivity at Infinite dilution for different Ions In an aqueous solution at 25X...

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