In our daily routine we come across variety of substances which are not pure but instead a homogeneous mixture of two or more pure substances. Many of these homogeneous mixtures have great importance in our life. The utility of these homogeneous mixtures depend largely on their compositions. Sometime individual components are same but they exhibit different properties in mixtures of different compositions. For example, presence of fluoride ions (F) in water at a concentration of about 1 ppm prevents tooth decay but if its concentration in water becomes 1.5 ppm, it becomes harmful and cause tooth to become mottled. The concentration of fluoride ion beyond this limit becomes poisonous (generally used for rat poisoning).
SOLUTE, SOLVENT AND SOLUTION
A solution is a homogeneous mixture of two or more pure substances whose composition
may be altered within certain limits. Though the solution is homogeneous in nature, yet it retains the properties of its constituents. The substances which make up the solution are generally called its components. The solution of two components is referred to as binary solution. Similarly, the solutions of three and four components are called ternary and quaternary solutions respectively: In this unit, we shall limit our discussion to binary solutions only. The two components of the binary solutions are respectively called a solvent and a solute.
Solvent is that component in the solution whose physical state is the same as that of the resulting solution while the other component is called solute. For solutions in which both the components have the same physical state as that of solution, the component which is in excess is called solvent and the other one is called solute.
For example, in case of solution of sugar and water, sugar is the solute and water is the solvent. Each component of binary solution may be in solid, liquid or gaseous state. Consequently, there can be nine different types of solutions as shown in Table 11.1.
Table 11.1. Different Types of Solutions
EXPRESSING CONCENTRATION OF SOLUTIONS OF SOLIDS IN LIQUIDS
Concentration of solution means describing its composition. Qualitatively, it can be described by using the word dilute for solutions having very small quantity of solute and the word concentrated for solutions having large quantity of solute. However, this kind of description leads to confusion. Therefore. quantitative description is more appropriate. Quantitatively, concentration of a solution refers to the amount of solute present in the given quantity of solution or solvent. The concentration of the solution may be expressed in any of the following ways:
1. Mass Percentage(% mass)
Mass percentage may be defined as the number of parts by mass of solute per hundred parts by mass of solution. For example, a 5% (aqueous) solution of sugar by mass means that 100 g of solution contain 5 g of sugar.
If W B be the mass of solute (B) and W A be the mass of solvent (A), then
Mass percentage of B = WB / WA + WB x 100
2. Volume Percentage(% volume)
This mode of concentration is used in case of solutions when solutes and solvents are both liquids. Volume percentage may be defined as the number of parts by volume of solute per hundred parts by volume of solution. For example, a 25% solution of ethyl alcohol (by volume) means that 100 cm3 of the solution contain 25 cm3 of ethyl alcohol and 75 cm3 of water.
If VA and VB be the volumes of component A and B, then
Volume percentage of B = VB + VA + VB x 100
3. Normality (N)
Normality of a solution is defined as the number of g-equivalents of the solute present in one litre (1 dm3) of the solution or milliequivalents of solute present in one cm3 of solution. It is represented by N.
Normality (N) = G- equivalents of solute (B) / volume of solution in litre
G-equivalents of solute (B) represents the ratio of its mass in gram (W B) to its gram-equivalent mass (GEM). Also, the ratio of the mass of solute in gram (W B) to the volume of the
solution in litre (V L) represents the strength of the solution.
Normality (N) = WB (g) / GEMB x V(L)
Strength of sol (gdm -3) / GEMB
WB (g) x 1000 / GEMB x V(cm3)
A solution having normality equal to unity is called a normal solution. Such a solution contains one gram equivalent of solute per litre of solution.
4. Molarity (M)
Molarity of a solution is defined as the number of gram mole of the solute present in one litre of the solution or millimoles of solute present in one cm3 of solution. It is represented by M.
Molarity (M) = Moles of solute / volume of solution in litres
= Mass of solute in gram / (Molar mass of solute) x (Volume of soln. (dm3))
Now, moles of the solute (n B) represents the ratio of mass of solute in gram (W B) to its molar mass (MMB). Thus, molarity can be represented by the expressions.
M = nB / V(L) or WB (g) / (MM)B x V(L)
or strength (gdm -3) / (MM)B OR WB (g) x 1000 / (MM )B x V (cm3)
A solution having molarity equal to unity is called molar solution. Such a solution contains one mole of solute per litre of solution. The solutions having molarity equal to 0.5 M, 0.1 M and 0.01 M are called semimolar, decimolar and centimolar solutions respectively. Molarity is expressed in units of mol L-1 or mol dm-3 It may be noted that bothnormality as well as molarity of a solution change with change in temperature.
5. Molality (m)
Molality of a solution may be defined as the number of gram mole of the solute present in 1000 g ( 1 kg) of the solvent. It is represented by m.
Mathematically, Molality (m)
= mole of solvent / mass of solvent of solvent ( kg)
= Mass of solute (g) / (Molar mass of solute) x (Mass of solvent (kg))
or m = nB / WA (Kg) or EB (g) / (MM)B x WA (kg)
or WB (g) x 1000 / (MM) B x WA (g)
A solution containing one mole of solute per 1000 g of solvent has molality equal to one and is called a molal solution. Molality is expressed in units of moles per kilogram (mol kg-1 ). The molality of a solution does not change with temperature.
6. Mole Fraction
Mole fraction may be defined as the ratio of number of moles of one component to the total number of moles of all the components (solvent and solute) present in the solution. It is denoted by the letter x followed by the subscript representing the component. Let us suppose that a solution contains the components A and B and suppose that W A g of A and W B g of B are present in it.
Moles of (nA) = WA / (MM)A
Moles of B (nB) = WB (MM)B
(MM) A and (MM)8 are molar masses of A and B respectively
Total number of moles of A and B = n A + n B
Mole fraction of A, (x A) = n A / n A + n B
Now, the sum of the mole fractions of solute and solvent in binary solution is unity.
This generalisation can be extended to solutions having more than two components by saying that the sum of mol fractions of all the components in a solution is always unity. It may be noted that the mole fraction is independent of the temperature.
Formality of a solution may be defined as the number of gram formula masses (moles) of the ionic solute present in one litre of the solution. It is represented by F.
Formality (F) = Number of moles of ions solute / Volume of solution in litres
= Mass of ionic solute (g) / (GFM) solute x V sol (dm 3)
Commonly, the term formality is used to express the concentration of the ionic solids which do not exist as molecules but exist as network of ions. A solution containing one gram formula mass of solute per litre of the solution has formality equal to one and is called formal solution. It may be mentioned here that like normality and molarity. The formality of a solution changes with change in temperature.
8. Parts Per Million (ppm)
When a solute is present in very minute amounts, its concentration is expressed in parts per million. It may be defined as the number of parts by mass of solute per million parts by mass of the solution. It is abbreviated as ppm.
Parts per million (ppm) = Mass of solute / Mass of solution x 106
Ppm = WB / WA + WB x 106