Methods of expressing concentration

Note very carefully there is no AG in this expression. This is because the standard values for 

Cu(s) formed and removed at the electrodes cancel, and the standard values for the Cu (aq) formed and removed at the electrodes cancel. This always happens with concentration cells. 

Note how important it is to distinguish between the solutions of CuS04(aq) in the left hand and right hand electrode compartments. 

Deduce the expression for the dependence of the emf on ionic strength for the following cell, 

Dealing with the term in the activity coefficients 23Q3RT. KAg+(aq)KFe +(aq) 2303RT f 

---

The table includes most of the methods of expressing concentration that are in current... 

Some analysts prefer to work with equivalents rather than moles. In that case, the normality method of expressing concentration is used and the equivalent weight is needed, rather than the formula weight. The equivalent weight of one substance reacts with the equivalent weight of the other substance. In other words, the reaction is always one-to-one one equivalent of one substance always reacts with one equivalent of the other. Thus, we can write the following as a true statement at the equivalence point of the titration ... 

A summary of the four methods of expressing concentration, together with a comparison of their relative advantages and disadvantages, is given in Table 11.3. 

The interesting part of this method of expressing concentration is that there is no consideration of the solvent or the solute identities. Note that 10 grams of anything that dissolves in 90 g of any solvent results in a 10% solution. The point is that the chemistry of a particular 10% solution is very likely to be different from another 10% solution because the number of molecules of solute is not the same in solutions produced and measured in this manner. 

Chemistry is a very wide field however, only a very small portion, indeed, of this seemingly complex subject is used in this book. These include equivalents and equivalent mass, methods of expressing concentrations, activity and active concentration, equilibrium and solubility product constants, and acids and bases. This knowledge of chemistry will be used under the unit processes part of this book. 

Equivalent concentration and normality. This method of expressing concentrations is analogous to molar concentrations, with the solute expressed in terms of equivalents. One problem that is often encountered is the conversion of a molar concentration to equivalent concentration. Let [C] be the molar concentration of any substance, where the symbol [ ] is read as the concentration of. Convert this to equivalent concentration. 

Method of Expressing Concentrations. The following methods are in common use to express concentrations of gases and vapours in air ... 

The above also illustrates the arbitrariness in the specification of Kp. The numerical value of the equilibrium constant would have been different if another pressure unit had been used or if some other method of expressing concentrations had been employed. Nevertheless, once the equilibrium constant has been determined for a particular set of physical parameters, this same value can be used to determine x for any other set of conditions at the same temperature. The above value of Kp holds so long as P is expressed in bars. It is this feature that renders the concept of equilibrium constants very useful. 

Approximately three fourths of the earth s surface is covered with water. The body fluids of all plants and animals are mainly water. Thus we can see that many important chemical reactions occur in aqueous (water) solutions, or in contact with water. In Chapter 3, we introduced solutions and methods of expressing concentrations of solutions. It is useful to know the kinds of substances that are soluble in water, and the forms in which they exist, before we begin our systematic study of chemical reactions. 

Because one mole of an acid does not necessarily neutralize one mole of a base, some chemists prefer a method of expressing concentration other than molarity to retain a one-to-one relationship. Concentrations of solutions of acids and bases are frequently expressed as normality (N). The normality of a solution is defined as the number of equivalent weights, or simply equivalents (eq), of solute per liter of solution. Normality may be represented symbolically as... 

Types of solutions and factors affecting solubility Methods of expressing concentration (The use of normalities is not tested.)... 

We must bear in mind the various methods of expressing concentration ... 

Several different methods of expressing concentration are commonly used in various branches of water chemistry and it is appropriate to present these here. This discussion will also provide definitions and methods of calculation that will be useful in the later chapters of this book,... 

There are two basic ways to express the mass concentration of dissolved species (solutes) in solution. The first is to state concentration in units of mass of solute in a unit volume of solution—the so-called w/v (weight/ volume) basis, The second is a w/w basis, that is, weight of solute in a given weight of solution. Both of these methods of expressing concentration are widely used in water chemistry. For example, the units mg/liter and ppm (parts per million) are, respectively, the w/v and w/w units most often used to express the concentration of various materials in waters and wastewaters. They can be interconverted if the density of the solution is known. 

The quantitative expression of the amount of dissolved solute in a particular quantity of solvent is known as the concentration of a solution. Several methods of expressing concentration are described in Section 14.6. 

The concentration of a solution expresses the amount of solute dissolved in a given quantity of solvent or solution. Because reactions are often conducted in solution, it s important to understand the methods of expressing concentration and to know how to prepare solutions of particular concentrations. The concentration of a solution may be expressed qualitatively or quantitatively. Let s begin with a look at the qualitative methods of expressing concentration. 

We need a method of expressing concentration that will easily indicate how many moles of solute are present per unit volume of solution. For this purpose, the concentration known as molarity is used in calculations involving chemical reactions. 

The concentration of a solute in a solution (i.e., the amount of a solute dissolved in a particular amount of solvent or in a particular amount of solution) is a vitally important concept in a variety of situations in real-world chemistry. For example, nitrate in drinking water is unsafe for infants. The amount of nitrate dissolved in a given amount of drinking water, or the concentration of nitrate in drinking water, is important to know in order to determine whether that water is safe for infants. Thousands of chemists and chemistry technicians in the United States analyze solutions such as drinking water daily to determine the quantity of a solute dissolved per unit volume of solution. This concentration level of a solute in a solution is thus a very important topic for students of chemistry. In this section, various methods of expressing concentration are defined and calculations associated with the determination of solution concentration are studied. 

---