Ways of Expressing Concentration

The concentration of a solute is the amount of solute dissolved in a given quantity of solvent or solution. The quantity of solvent or solution can be expressed in terms of volume or in terms of mass or molar amount. Thus, there are several ways of expressing the concentration of a solution. 

Recall that the molarity of a solution is the moles of solute in a liter of solution.  

For example, 0.20 mol of ethylene glycol dissolved in enough water to give 2.0 L of solution has a molarity of 

The solution is 0.10 molar (denoted M). This unit is especially useful when you wish to dispense a given amount of solute, because the unit directly relates the amount of solute to the volume of solution. 

Some other concentration units are defined in terms of the mass or molar amount of solvent or solution. The most important of these are mass (or weight) percentage of solute, molality, and mole fraction. 

---

To indicate the composition of a particular solution we must show the relative amounts as well as the kind of components. These relative amounts chemists call concentrations. Chemists use different ways of expressing concentration... 

There are numerous ways of expressing concentration in diffusion problems, the most important for our purposes being mass density, molar density, mass fraction, and mole fraction. The chemical engineer and the chemist are familiar with the relationships between these quantities. Table I is given for the sake of summarizing the notation used here. 

Another way of expressing concentrations that is used commonly with gases (see p 162) and colligative properties (see p 328) is mole fraction, which is defined (for a given component) as being the moles of component in question divided by the total moles of all components in solution. For a solution that has three components (A, B, and C), the mole fraction of A is given by... 

In daily life, it s often sufficient to describe a solution as either dilute or concentrated. In scientific work, though, it s usually necessary to know the exact concentration of a solution—that is, to know the exact amount of solute dissolved in a given amount of solvent. There are many ways of expressing concentration, each of which has its own advantages and disadvantages. We ll look briefly at four of the most common methods molarity, mole fraction, mass percent, and molality. 

The most common way of expressing concentration in a chemistry laboratory is to use molarity. As discussed previously in Section 3.7, a solution s molarity is given by the number of moles of solute per liter of solution (mol /L, abbreviated M). If,... 

This is a way of expressing concentration. One molar (1m) concentration contains the mass one mole in grams dissolved in 11 (1 dm3) of solution. 

The most common way of expressing concentration in chemistry is molarity. Molarity is the ratio of moles of solute to total liters of solution ... 

Molality Way of expressing concentration. Ratio of moles of solute to kilograms of solvent. 

In a solution, the solute is distributed evenly throughout the solvent. This means that any part of a solution has the same ratio of solute to solvent as any other part of the solution. This ratio is the concentration of the solution. Some common ways of expressing concentration are given in Table i. 

We reviewed some of the ways of expressing concentrations earlier in the chapter. Basic concepts are summarized below. 

A measure of concentration frequently used for aqueous solutions is % (w/v), a ratio of solute weight (actually mass) to solution volume. Thus, a 1.5% (w/v) NaCl solution contains 1.5 g of NaCl per 100. mL of solution. This way of expressing concentrations is particularly common in medical labs and other health-related facilities. 

All the terms we just discussed represent different ways of expressing concentration, so they are interconvertible. Keep these points in mind ... 

When a solution is heated, which ways of expressing concentration change in value Which remain unchanged Explain. 

Concentration is the amount of one substance found in a given volume of another substance. Depending on the materials involved, there are many different ways of expressing concentration. Two of the most common ways are ppm (parts per million) and milligrams per kilogram (mg/kg). Toxicology will be discussed further in Chapter 8. Examples of some 2.3 poison gases are fluorine, chlorine. 

Why do we need any other way of expressing concentration A statement of concentration w/v is a good recipe for preparing a solution, but it does not tell us anything directly about how many molecules of the substance we have dissolved. Since we know that chemical reactions involve exact numbers of molecules of each compound reacting in a precise ratio, it would be helpful to build that information in somehow. This is done by defining the molar concentration (Table III.l). This has nothing to do with teeth. It is the number of moles of a substance per litre. But what is a mole in this context We have already met the idea of relative molecular... 

Table III.l Different ways of expressing concentration Type of concentration Typical units...

We examine several common ways of expressing concentration, including mole fraction, molarity, and molality. 

If, in addition to the quantities of solute and solution, the solution density is known, the various ways of expressing concentration are interconvertible. 

---