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In chemistry, it is common to work with different units of concentration, and morality and normality are two of the most frequently used. On the one hand, molarity is a chemical unit of concentration that indicates the number of moles of solute in each liter of solution . On the other hand, normality is also a unit of chemical concentration , but expressed in terms of the number of solute equivalents per liter of solution .
Although it may not seem like it, normality and molarity are closely related, since the number of moles and equivalents are also. However, there are a number of very important differences that make each unit more practical or useful for different applications. For this reason, in this article, the difference between molarity and normality will be covered, what each of these concentration units is used for, how they are calculated, how to convert from one concentration unit to the other, and in which situations it is more convenient to use one or the other.
molarity
As mentioned at the beginning, molarity is a chemical unit of concentration in which the amount of solute is expressed in terms of the number of moles and the volume of the solution in liters. It is one of the most used concentration units since it allows to know very easily and quickly the amount of solute present in any volume of solution.
Molarity is expressed in units of mol/L, which is often read as “molar.” Thus, a concentration of 0.5 mol/L is usually read as 0.5 molar.
Formulas to calculate molarity
The formula that defines molarity is:
where n solute represents the number of moles of solute and V solution represents the volume of the solution expressed in liters. However, it is very common to replace the number of moles by its formula which is given by the mass divided by the molar mass of the solute to give the following formula:
When should you use molarity?
Molarity is a general-purpose unit of concentration, which means it works for almost any situation involving solutions, as long as there are no large changes in temperature.
The latter is because temperature can affect the volume of a solution, causing molarity, which depends on volume, to also vary with temperature. In these cases, it is preferable to use another unit of concentration that is expressed in terms of mass or amount of matter, such as molality or mole fractions.
Normal
Normality is also a unit of chemical concentration. The main difference between normality and molarity is that the former expresses the amount of solute in terms of the number of equivalents instead of moles.
The big problem with normality for most people is that, unlike molarity, the same solution can have more than one normality, since the concept of the number of equivalents depends on what the solute is used for or in what way. what types of chemical reactions it will participate in.
Formulas to calculate normality
The formulas for calculating normality are very similar to those for molarity. The mathematical form of the definition of normality is:
where n eq. solute represents the number of solute equivalents and V solution represents the volume of the solution expressed in liters. To calculate normality from the mass of the solute, there is also a formula similar to that for molarity:
Where PE solute (the equivalent weight of the solute) represents the weight in grams of 1 equivalent of solute. This is given by the molar mass divided by an integer that represents the number of equivalents per mole of the substance, and which we will call ω (the Greek letter omega) to avoid confusing it with the true number of equivalents (n eq ) .
Combining this equation with the previous one, we get:
The concept of the number of equivalents
The key to understanding the concept of the number of equivalents, and indeed the reason that “normal” concentration or normality is so called, lies in ω. This number depends on the use to which the solute is put or the chemical reaction in which it will participate.
For each type of major chemical reaction that involves at least two chemical substances, we can define what we will call the “Normal” reactant, which is nothing more than a generic term that we use to identify the reactant that participates in the simplest possible version of the type. particular reaction.
For example , if we are talking about an acid-base reaction , the simplest case would be one in which any monoprotic acid (HA) reacts with a monobasic base (B), to give the respective conjugate pairs:
The monoprotic acid HA and the monobasic base B are what we would call a normal acid and base, respectively. This means that any acid such as HCl or HNO 3 is a normal acid, and any base such as NaOH or NH 3 would be an example of a normal base.
If we now consider an acid such as sulfuric acid (H 2 SO 4 ) that is diprotic, the reaction with a normal base would be:
As we can see, each mole of this acid is “equivalent” to 2 moles of a normal acid . Therefore, we say that the number of equivalents per mole of sulfuric acid is 2. For this reason, a 0.1 molar solution of H 2 SO 4 is equivalent to a 0.2 molar solution of a normal acid, so we say that the normality of such a solution is 0.2.
In other words, we can redefine the concept of normality as the molar concentration that a normal reactant would have participating in the same type of chemical reaction as the solute .
The following table shows how ω is determined for each type of solute, depending on the reaction in which it will be involved:
| type of chemical reaction | reagent type | Number of equivalents per mole (ω) |
| Reactions involving salts | You go out | ω is given by the total number of positive or negative charges in the neutral salt (both numbers are the same). It is calculated by multiplying the number of cations by their charge or the number of anions by theirs. |
| Acid Base Reactions | acids | ω is given by the number of hydrogens that give up in the reaction. |
| Bases | ω is given by the number of hydrogens that it can capture | |
| Redox reactions | oxidizing agent | ω is given by the number of electrons captured by each molecule of oxidizing agent in the balanced reduction half-reaction. |
| reducing agent | ω is given by the number of electrons given up by each molecule of reducing agent in the balanced oxidation half-reaction. | |
| Solutes that do not participate in reactions | ——- | ω is worth 1eq/mol |
When should you use normality?
Unlike molarity which is often used in any context, normality is mainly used in situations involving chemical reactions in solution, as they facilitate stoichiometric calculations without the need to write balanced or adjusted chemical reactions.
Because of the way the number of equivalents per mole is defined, the number of equivalents of one reactant will always equal the number of equivalents of the other when they react in stoichiometric ratios. Since the number of equivalents can be easily found from the normality and the volume of solution, we can carry out stoichiometric calculations very quickly without worrying about the details of the reaction.
This is particularly practical in volumetric titrations or titrations, since, at the equivalence point of the titration, it will always be true that:
And substituting the equivalents by the product of the normality by the volume, we obtain:
Something similar could be done with molarity, but it inevitably requires that we write the chemical equation and adjust it to obtain the necessary stoichiometric ratios.
Conversion between molarity and normality
Converting between molarity and normality is very easy, since the second is always an integer multiple of the first as shown below:
If we know the molarity of a solution, we can calculate its various normalities simply by multiplying the molarity by the respective number of equivalents per mole, ω.
