Stoichiometric calculations acid-base reactions

Quantitative Calculations In acid-base titrimetry the quantitative relationship between the analyte and the titrant is determined by the stoichiometry of the relevant reactions. As outlined in Section 2C, stoichiometric calculations may be simplified by focusing on appropriate conservation principles. In an acid-base reaction the number of protons transferred between the acid and base is conserved thus... 

Stoichiometric calculations for redox reactions in water solution are carried out in much the same way as those for precipitation reactions (Example 4.5) or acid-base reactions (Example 4.7). 

In an acid-base titration, you carefully measure the volumes of acid and base that react. Then, knowing the concentration of either the acid or the base, and the stoichiometric relationship between them, you calculate the concentration of the other reactant. The equivalence point in the titration occurs when just enough acid and base have been mixed for a complete reaction to occur, with no excess of either reactant. As you learned in Chapter 8, you can find the equivalence point from a graph that shows pH versus volume of one solution added to the other solution. To determine the equivalence point experimentally, you need to measure the pH. Because pH meters are expensive, and the glass electrodes are fragile, titrations are often performed using an acid-base indicator. 

The stoichiometric calculations of Chapters 12 and 13 are based on the mole as the fundamental chemical unit in reactions. An alternative method of calculation utilizes the equivalent as a fundamental chemical unit. There are two kinds of equivalents, the type depending on the reaction in question we shall refer to them as acid-base equivalents (or simply as equivalents) and electron-transfer equivalents (or E-T equivalents). The concept of an equivalent is particularly useful when dealing with complex or unknown mixtures, or when working out the structure and properties of unknown compounds. In addition, it emphasizes a basic characteristic of all chemical reactions that is directly applicable to all types of titration analyses. 

When a strong acid or base is added to a buffered solution, it is best to deal with the stoichiometry of the resulting reaction first. After the stoichiometric calculations are completed, then consider the equilibrium calculations. This procedure can be represented as follows ... 

Titrations of weak bases with strong acids can be treated using the procedures we have introduced previously. As always, you should first think about the major species in solution and decide whether a reaction occurs that runs essentially to completion. If such a reaction does occur, let it run to completion and then do the stoichiometric calculations. Finally, choose the dominant equilibrium and calculate the pH. 

This reaction illustrates a very important general principle The hydroxide ion is such a strong base that for purposes of stoichiometric calculations it can be assumed to react completely with any weak acid that we will encounter. Of course, OH ions also react completely with the ions in solutions of strong acids. 

Do the problem in two parts. First, you assume that the H30 ion from the strong acid and the conjugate base from the buffer react completely. This is a stoichiometric calculation. Actually, the HgO ion and the base from the buffer reach equilibrium just before complete reaction. So you now solve the equilibrium problem using concentrations from the stoichiometric calculation. Because these concentrations are not far from equilibrium, you can use the usual simplifying assumption about x. 

For the calculation of a mono-protic acid with a mono-basic base, the stoichiometry is simply 1 1 because 1 mol of acid reacts with 1 mol of base. We say the stoichiometric ratio s = l. The value of s will be two if sulphuric acid reacts with NaOH since 2 mol of base are required to react fully with 1 mol of acid. For the reaction of NaOH with citric acid, s = 3 and s = 4 if the acid is H4EDTA. 

STRATEGY First, we find the volume of titrant needed to reach the stoichiometric point. At this point, pH = 7. After the stoichiometric point, the amount of acid added exceeds the amount of base in the analyte, and we expect a pH of less than 7. We use the reaction stoichiometry to determine how much of the acid added remains after neutralization. Then we use the total volume of solution to find the molar concentration of H30+ and convert it to pH. To implement the calculation, work through the steps in Example 11.5. 

A mathematical model was used to calculate the concentrations of all components and the pH at every equilibrium stage. The pH and concentrations of the components at every equilibrium stage were predicted with reasonable accuracy. This model is based on dissociation and reaction equilibria of the compounds, stoichiometric balances and an electroneutrality equation. Precipitation of 6-aminopenicillanic acid, which was observed at a combination of low pH and high 6-APA concentration in the aqueous phase, is not taken into account in the model. 

The oxycyanide reaction on which the above method is based is not stoichiometric so an excess of reagent must be used. The 20 ml specified is sufficient for the titration of up to 12 mg of chloride but it should be increased to 30 ml if this quantity is exceeded. It should be noted that, at the end-point of the titration with sodium chloride solution, the volumes and temperatures of both the test and comparison solutions should be the same. For many practical purposes it may be sufficiently accurate to calculate the chlorine content of the sample from the volume of 0-02N sulphuric acid used, thus dispensing with the second titration. 

The Stoichiometry Calculation As the added acid is neutraUzed, it converts a stoichiometric amount of the base into its conjugate acid through the neutralization reaction (Figure 16.3a on p. 762) ... 

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