Concentration Terms - Explanation, Notations and Formulae for JEE

Before we venture further into the topic, we must first understand what concentration terms are. Then we’ll be looking at the relation between concentration terms. Concentration terms, which are mostly used in Chemistry and other related fields, essentially refer to the idea or the concept of how much of a particular substance is mixed with a definite quantity of another, to get a specific mixture in the end. In Chemistry, this term comes up multiple times, especially in the context of mixtures, mixing ratios and solutions. Solutions are a mixture in which the solute is dissolved in the solvent.

If you want to increase the concentration of the solution, you must add more solute. Similarly, if you want to lower the concentration of the solution, you must add more solvent to make it dilute. The solution reaches its saturation point when the solvent cannot dissolve the added solute any further. The extra solute does not dissolve and what remains is a suspension. The saturation point is dependent upon multiple factors, both physical and chemical. Temperature and pressure are two physical factors which can greatly affect the quantity of solute that a specific quantity of the solvent can dissolve.

On the other hand, the chemical properties like the nature of the solvent, the characteristic properties of the solute and their density, can also affect the quantity of solute a certain amount of solvent can dissolve. The concentration of a mixture or a solution can be represented in two ways. The qualitative expression is also known as the informal method, while the quantitative representation is numerical in nature.

Qualitative Notation of Concentration Terms

When we express the concentration of a mixture or a solution qualitatively, we use terms like ‘dilute’ and ‘weak’. Similarly, to express the fact that a solution is high in concentration of the solute, we use terms like ‘concentrated’. For coloured solutions, which are also termed as chromatic solutions, the colour of the mixture deepens and becomes darker when the mixture is highly concentrated. Conversely, as the solution becomes more and more dilute, the colour fades away. This notation does not have any concentration term formulas.

Quantitative Notations of Concentration Terms

From the perspective of Chemistry, quantitative notations are a lot more useful since they give us not only an idea of the concentration, but also the ratios of the solute and the solvent that make up the solution or mixture. Measuring the concentration of a solution depends largely on the volumes of the substances taken into consideration. This again depends on factors like ambient temperature, pressure and other physical conditions. Normally, the convention is to measure the concentration at 1 ATM at pressure and $25^{\circ}$ Celsius temperature. There are many notations that come under this heading. Each term has its own importance. 

• Mass Percentage: The mass percentage is the mass of the solute and the solvent as a percentage of the mass of the resultant solution. For example: if in a bucket, there is 40 kg of milk and 60 kg of water, then the mass percentage of the milk will be 40%  and that of the water will be 60%. The mass percentage was previously referred to as the weight-by-weight percentage. When a solution or a mixture of any nature is being sold in a commercial capacity, the aqueous reagents are often expressed in terms of their mass percentage, along with their specific gravity. 

• Mass-Volume Percentage: As the name suggests, the mass-volume percentage is the mass of either the solute or the solvent, or both, as a percentage of the total volume of the mixture. Denoted as $\%~\dfrac{m}{v}$ or $\%~\dfrac{w}{v}$, the mass percentage notation is also known as the weight-volume percentage. In cases of solutions which involve solid reagents, mass-volume percentage is probably the best terminology to use, since it gives you an idea of the mass as well as the volumes concerned.  According to the formula, this is the weight of the solute (in grams) multiplied by 100 and divided by the volume of the mixture in millilitres. 

• Volume-Volume Percentage: The volume-volume percentage or $\%~\dfrac{v}{v}$ gives us the volume of the solute in millilitres, present in every 100 ml of the solution. In the case of mixtures where two liquids are being dissolved or mixed, the volume-volume percentage provides us with the best notations. For example: if we say the mixture A has 10 ml % of water, it would mean that every 100 ml of the solution A will have 10 ml of water.

• Molarity (M): Molarity is the notation, where the concentration is expressed in terms of the number of moles of a given solute that is present in 1 litre of the solution. The formula of molarity is $M=\dfrac{n}{v}$, where n is the number of moles of the solute and v is the quantity of solution in litres. Molarity is one of the most accurate ways of expressing the concentration, since it gives us the exact number of particles or units present in the mixture; irrespective of the weight, volume and density. In fact, molarity is sometimes preferred over stoichiometric calculations owing to its accuracy. For example: if 10 litres of a solution A has 5 moles of dissolved solute, then the molarity of the solution will be 0.5 molar or 0.5 M. 

• Molality (m): Just as molarity expresses the number of moles present with respect to the volume, molality gives us the number of moles present in 1 kg of the solution.The general formula of molality is $m=\dfrac{n}{w}$, where n is the number of moles of the solute and w is the weight of the solvent in kg. Since there is no volume involved here, even if it changes due to the physical factors, the concentration term remains unchanged. This is an advantage that molarity does not have. For instance: if 5 moles of the solute is present in 10 kg of the solvent, then the molality of the solution will be 0.5 m or 0.5 molal.

• Normality (N): Normality has its use in the field of acids and bases. In the case of chemical reactions, normality is expressed as the mass of the acid or the base that is capable of donating exactly 1 mole of protons or hydrogen ions. This term is also used extensively for acid base reactions. The general formula is given by $N=\dfrac{eq}{v}$, where eq is the number of gram equivalence and v is the volume of the solvent in litres. 

• Formality (F): One of the least used concentration notations, formality is calculated by taking the formula masses of the chemicals present in 1 L of the solution. Since it takes only the original components, it is less accurate and therefore less used than the other concentration terms. Formality is represented by ‘F’.

Parts-Per Notations of Concentration

The parts-per notations are used for lower concentrations, especially where the number of particles or units present play an important role. Normally, these terms are used for trace elements present in various sections. The following table gives the concentration term formula:

Conclusion

Concentration terms basically refer to the different notations used to express the concentration of a solution or a mixture, with either aqueous, solid or gaseous reagents. They can be expressed in three different ways: the qualitative notation, quantitative notation and parts-per notations. The quantitative expressions include molarity, normality, mass-volume percentage and volume-volume percentage to name a few. The relationship between concentration terms is useful for measurement in different branches of science and otherwise.