Question

Different modes of expressing concentration of an aqueous solution

Answer 1

of H2OH2O under these conditions is very close to 1.0 L, and a 0.50 M solution of KBrKBr in water, for example, has approximately the same concentration as a 0.50 m solution.

Another common way of describing concentration is as the ratio of the mass of the solute to the total mass of the solution. The result can be expressed as mass percentage, parts per million (ppm), or parts per billion (ppb):

mass percentage=mass of solutemass of solution×100(13.4.2)(13.4.2)mass percentage=mass of solutemass of solution×100

parts per million (ppm)=mass of solutemass of solution×106(13.4.3)(13.4.3)parts per million (ppm)=mass of solutemass of solution×106

parts per billion (ppb)=mass of solutemass of solution×109(13.4.4)(13.4.4)parts per billion (ppb)=mass of solutemass of solution×109

In the health sciences, the concentration of a solution is often expressed as parts per thousand (ppt), indicated as a proportion. For example, adrenalin, the hormone produced in high-stress situations, is available in a 1:1000 solution, or one gram of adrenalin per 1000 g of solution.


What is the molarity of Pb2+Pb2+ in a 9.0 ppb aqueous solution?

Use your calculated concentration to determine how many grams of Pb2+Pb2+ are in an 8 oz glass of water.

Answer aAnswer b

How do chemists decide which units of concentration to use for a particular application? Although molarity is commonly used to express concentrations for reactions in solution or for titrations, it does have one drawback—molarity is the number of moles of solute divided by the volume of the solution, and the volume of a solution depends on its density, which is a function of temperature. Because volumetric glassware is calibrated at a particular temperature, typically 20°C, the molarity may differ from the original value by several percent if a solution is prepared or used at a significantly different temperature, such as 40°C or 0°C. For many applications this may not be a problem, but for precise work these errors can become important. In contrast, mole fraction, molality, and mass percentage depend on only the masses of the solute and solvent, which are independent of temperature.

Mole fraction is not very useful for experiments that involve quantitative reactions, but it is convenient for calculating the partial pressure of gases in mixtures, as discussed previously. Mole fractions are also useful for calculating the vapor pressures of certain types of solutions. Molality is particularly useful for determining how properties such as the freezing or boiling point of a solution vary with solute concentration. Because mass percentage and parts per million or billion are simply different ways of expressing the ratio of the mass of a solute to the mass of the solution, they enable us to express the concentration of a substance even when the molecular mass of the substance is unknown. Units of ppb or ppm are also used to express very low concentrations, such as those of residual impurities in foods or of pollutants in environmental studies.
Table 13.4.113.4.1 summarizes the different units of concentration and typical applications for each. When the molar mass of the solute and the density of the solution are known, it becomes relatively easy with practice to convert among the units of concentration we have discussed, as illustrated in