Calculations involving solutions

Stoichiometric calculations involving solutions of specified molar concentration are usually quite simple since the number of moles of a reactant or product is simply volume x molar concentration. 

Sometimes, the calculation involves a monoprotic acid and a dihydroxy base or another set of conditions in which the relationship is not 1 1. We have to keep track of the various concentrations so that the molarities do not get mixed up. However, stoichiometric calculations involving solutions of specified normalities are even simpler. By the definition of equivalent mass in Chapter 12, two solutions will react exactly with each other if... 

Recall that stoichiometry involves calculating the amounts of reactants and products in chemical reactions. If you know the atoms or ions in a formula or a reaction, you can use stoichiometry to determine the amounts of these atoms or ions that react. Solving stoichiometry problems in solution chemistry involves the same strategies you learned in Unit 2. Calculations involving solutions sometimes require a few additional steps, however. For example, if a precipitate forms, the net ionic equation may be easier to use than the chemical equation. Also, some problems may require you to calculate the amount of a reactant, given the volume and concentration of the solution. 

This experiment supports classroom discussions concerning stoichiometric calculations involving solutions. It also introduces the students to the use of a spectrophotometer to measure turbidity. The potential problems mentioned by Professor Preesip provide the opportunity to turn it into a research experiment. 

A cautionary note In Chapter 15 (p. 600) we assumed that dissolved substances exhibit ideal behavior for our calculations involving solution concentrations, but this assumption is not always valid. For example, a solution of barium fluoride (BaF2) may contain both neutral and charged ion pairs, such as Bap2 and BaF, in addition to Ba + and F ions. Furthermore, many anions in the ionic compounds listed in Table 16.2 are conjugate bases of weak acids. Consider copper sulfide (CuS). The ion can hydrolyze as follows... 

So far we have considered only the volume as a partial molar quantity. But calculations involving solutes will require knowledge of all the thermodynamic properties of dissolved substances, such as H, S, Cp, and of course G, as well as the pressure and temperature derivatives of these. These quantities are for the most part derived from calorimetric measurements, that is, of the amount of heat released or absorbed during the dissolution process, whereas V is the result of volume or density measurements. 

Be aware that the definition of pH just shown, and indeed aU the calculations involving solution concentrations (expressed either as molarity or molality) discussed in previous chapters, are subject to error because we have implicitly assumed ideal behavior, hi reality, ion-pair formation and other types of intermolecular interactions may affect the actual concentrations of species in solution. The situation is analogous to the relationships between ideal gas behavior and the behavior of real gases discussed in Chapter 5. Depending on temperature, volume, and amount and type of gas present. 

Guide to Calculations Involving Solutions in Chemical Reactions... 

Calculating Solution Concentration 406 Using Concentration to Calculate Mass or Volume 409 Calculating Dilution Quantities 413 Calculations Involving Solutions in Chemical Reactions 414 Calculating Molality 420 Using Molality 422... 

It should be noted that, in calculations involving solution equilibria, certain rules should always be considered. 

We will consider a number of additional examples of stoichiometric calculations involving solutions in Chapter 5. 

In an electric field, the mobility of each ion is reduced because of the attraction or drag exerted by its ionic atmosphere. Similarly, the magnitudes of colligative properties are reduced. This explains why, for example, the value of i for 0.010 m NaCl is 1.94 rather than 2.00. What we can say is that each type of ion in an aqueous solution has two "concentrations." One is called the stoichiometric concentration and is based on the amount of solute dissolved. The other is an "effective" concentration, called the activity, which takes into account interionic attractions. Stoichiometric calculations of the type presented in Chapters 4 and 5 can be made with great accuracy using stoichiometric concentrations. However, no calculations involving solution properties are 100% accurate if stoichiometric concentrations are used. Activities are needed instead. The activity of an ion in solution is related to its stoichiometric concentration through a factor called an activity coefficient. Activities were introduced in Chapter 13. In Chapter 15 their importance in chemical equilibrium will be discussed in more detail. 

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