Stoichiometric calculations standard solution

The volume of standard solution that reacts with the substance in the test solution is accurately measured. This volume, together with a knowledge of concentration of the standard solution and the stoichiometric relationship between the reactants, is used to calculate the amount of substance present in the test solution. Specific examples of the different types of calculations involved are shown in Chapters 22 to 25. 

In this section, we describe two types of volumetric calculations. The first involves computing the molarity of solutions that have been standardized against either a primary-standard or another standard solution. The second involves calculating the amount of analyte in a sample from titration data. Both types are based on three algebraic relationships. Two of these are Equations 13-1 and 13-3, both of which are based on millimoles and milliliters. The third relationship is the stoichiometric ratio of the number of millimoles of the analyte to the number of millimoles of titrant. 

From changes in free energy in standard reference conditions it is possible to calculate equilibrium constants for reactions involving several reactants and products. Consider, for example, the chemical reaction aA + bB = cC + dD at equilibrium in solution. For this reaction we can define a stoichiometric equilibrium constant in terms of the concentrations of the reactants and products as... 

Since AG° can be calculated from the values of the chemical potentials of A, B, C, D, in the standard reference state (given in tables), the stoichiometric equilibrium constant Kc can be calculated. (More accurately we ought to use activities instead of concentrations to take into account the ionic strength of the solution this can be done introducing the corresponding correction factors, but in dilute solutions this correction is normally not necessary - the activities are practically equal to the concentrations and Kc is then a true thermodynamic constant). 

This is an example of the method of back titration, in which more acid (HCI) is added than is necessary to stoichiometrically react with the base (Mg(OH)2), in order to be certain that all the base has reacted. One then titrates the excess acid with a standardized base solution (NaOH) and in a series of calculations, determines the amount of unknown base (Mg(OH)2). 

A titrimetric method involves the controlled reaction of a standard reagent in known amounts with a solution of the analyte, in order that the stoichiometric or equivalence point for the reaction between the reagent and the analyte may be located. If the details of the reaction are known and the stoichiometric point is located accurately and precisely, the amount of analyte present may be calculated from the known quantity of standard reagent consumed in the reaction. In most cases a standard reagent solution is prepared and added manually or automatically from a burette an alternative procedure is coulometric generation of the reagent in situ. The stoichiometric point may be detected by use of a visual indicator or by an electrochemical method (Chapter 6). 

If there is no agreement in calculated and observed solid-solution properties we can only conclude that equilibrium was not established. The validity of the provisional activity coefficients depends on the validity of the original assumption that stoichiometric saturation was established. If independent data for the standard free energy of formation of the solid... 

From a Solution Model. Calculation of the difference in reduced standard-state chemical potentials by methods I or III in the absence of experimental thermodynamic properties for the liquid phase necessitates the imposition of a solution model to represent the activity coefficients of the stoichiometric liquid. Method I is equivalent to the equation of Vieland (106) and has been used almost exclusively in the literature. The principal difference between methods I and III is in the evaluation of the activity coefficients... 

The technique is generally unaffected by the state (ionic, imdissociated, sometimes complexed) of the analyte to be titrated. For example, the direct potentiometric determination of pH in a solution of a weak acid reports only the hydrogen ion concentration. Since the major portion of the acid is present in the undissociated form, direct potentiometry can not provide data yielding the total acid concentration. Potentiometric titration involves titrating the acid solution with a standard base, determining the equivalence point volume of standard base solution used, and calculating the total weak acid concentration from the stoichiometric data. 

The calculation of the pH of a solution of a salt of a weak base/strong acid uses exactiy the same procedure as does the calculation of the pH of a solution of a weak acid. The same considerations apply. For cases where the p fa lies in the range 4.0 to 10.0, then the two standard approximations apply unless the stoichiometric concentration is very low. For low concentrations it is likely that the self ionisation of water has to be considered. This is also necessary for the case of a salt of a weak base/strong acid where the conjugate acid has a pA a greater than around 10.0. 

C, 1 bar is 10 bar. The pyrrhotite in this equilibrium is Feo.gaS, which may be considered as a solid solution composition in the system FeS — Sa. The activity of FeS in this pyrrhotite is 0.46 based on a standard state of pure stoichiometric FeS at the same P and T. The pyrite is pure stoichiometric FeSa. Calculate ArG° for the reaction forming pyrite from pyrrhotite and Sa gas at this P, T. 

Strategy The aim is to find the values of and v corresponding to the reaction, for then we can use a modified form of eqn 5.16 to calculate the value of K in neutral solution from fd. To do so, we express the equation as the difference of two reduction half-reactions. The stoichiometric number of the electron in these matching half-reactions is the value of v we require. We then look up the biological standard potentials for the couples corresponding to the half-reactions and calculate their difference to find... 

Many of the experimental results presented in this chapter were obtained in ferroin-catalyzed BZ systems prepared according to the following standard procedure. Solutions were prepared with reagent-grade chemicals and distilled water. The three initial reactant solutions were sodium bromate in sulfuric acid, sodium bromide in water, and malonic acid in water. A 25 mM solution of ferroin, the tris(l, 10-phenanthroline) ferrous sulfate complex, was prepared by dissolving stoichiometric amounts of phenanthroline and ferrous sulfate in 25 mM sulfuric acid. All solutions were filtered through 0.44-jim Millipore filters and stored in separate containers. Concentrations were calculated from the weights of the dissolved chemicals. 

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