Pure water at a particular temperature will always have the same conductivity. The conductance of a sample of pure water depends on how the measurement is made—how big a sample, how far apart the measuring electrodes are, etc.
It is defined as the reciprocal of the resistance in ohms, measured between the opposing faces of a 1 cm cube of liquid at a specific temperature.
See Conductivity Cell Figure. The basic unit of conductance is the Siemens S. Because a measurement gives the conductance, techniques have been worked out to convert the measured value to the conductivity, so that results can be compared from different experiments.
This is done by measuring a cell constant K for each setup, using a solution of known conductivity. The cell constant is related to the physical characteristics of the measuring cell. K is defined for two flat, parallel measuring electrodes as the electrode separation distance d divided by the electrode area A. Thus, for a 1 cm cube of liquid In practice, the measured cell value is entered into the meter, and the conversion from conductance to conductivity is done automatically.
The K value used varies with the linear measuring range of the cell selected. For some solutions, such as pure water, the conductivity numbers are so low that some users prefer to use resistivity and resistance instead. From Eq. Weak acids have a higher pH than strong acids, because weak acids do not release all of its hydrogens. The acid dissociation constant tell us the extent to which an acid dissociates.
This equation is used fairly often when looking at equilibrium reactions. During equilibrium, the rates of the forward and backward reaction are the same. However, the concentrations tend to be varied. Since concentration is what gives us an idea of how much substance has dissociated, we can relate concentration ratios to give us a constant.
K is found by first finding out the molarity of each substance. Then, just as shown in the equation, we divide the products by the reactants, excluding solids and liquids. Also when there is more than one product or reactant, their concentrations must be multiplied together. Even though you will not see a multiplication sign, if there are two molecules associated, remember to multiply them.
If there is a coefficient in front of a molecule, the concentration must be raised to that power in the calculations. There is more of the acid, and less of the ions.
You can think of Ka as a way of relating concentration in order to find out other calculations, typically the pH of a substance. A pH tells you how basic or acidic something is, and as we have learned that depends on how much ions become dissociated.
Calculate the ionization constant of a weak acid. Solve for K a given 0. The quantitative treatment of these effects was first worked out by P. Debye and W. Huckel in the early 's, and was improved upon by Ostwald a few years later.
This work represented one of the major advances in physical chemistry in the first half of the 20th Century, and put the behavior of electrolytic solutions on a sound theoretical basis. Even so, the Debye-Huckel theory breaks down for concentrations in excess of about 10 —3 M L —1 for most ions.
The curvature of the plots for intermediate electrolytes is a simple consequence of the Le Chatelier effect , which predicts that the equilibrium. In more dilute solutions, the actual concentrations of these ions is smaller, but their fractional abundance in relation to the undissociated form is greater. Dissociation, of course, is a matter of degree. The equilibrium constants for the dissociation of an intermediate electrolyte salt MX are typically in the range of
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