9 junio, 2024

Normal solutions: concept, preparation, examples

The normal solutions are all those in which the concentration of the solute is expressed in equivalents or equivalent-grams per liter of solution.

When talking about the normality of a solution, it refers to the number of equivalents of a solute that it has per liter of solution. But to find this number of equivalents it is necessary to know its equivalent weight, which varies between the elements, the type of chemical compound, or even the reaction that takes place.

This is why normal solutions are generally more complicated to prepare in terms of their theoretical calculations. They are recognized because they present the ‘N’ for normality on their labels. Many acids and bases have been prepared according to this concentration; for example, 0.01 N NaOH.

Where most normal solutions occur are in the reagents used for redox reactions. Generally, they are salt solutions such as KMnO4, CuSO4, CrCl3, among others.

Most of the time, and in general terms, molar solutions are preferred over normal ones. This is because the former are easier to prepare and to relate their concentrations to any process.

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Preparation of normal solutions

How are normal solutions prepared? Although the steps to follow are not different from those of other solutions, they will be explained below:

Step 1

Find the chemical characteristics of the reagent that you want to prepare, using the information that appears on the label of the reagent containers. The information required is the chemical formula of the reagent, its molecular weight, whether the reagent is anhydrous or not, etc.

Step 2

Perform the necessary calculations for the preparation of normal solutions. Normality is expressed in equivalents per liter (Eq/L) and is abbreviated with the letter ‘N’.

The calculation begins by dividing the concentration of the solution expressed in grams/liter (g/L) by the equivalent weight expressed in grams per equivalent (g/Eq). But first, the equivalent weight of the reagent must be obtained, taking into account the type of chemical reagent.

Example

How many grams of sodium carbonate are needed to prepare one liter of a 2N solution, knowing that it has a molecular weight of 106 g/mol?

By definition, a normal solution (N) is expressed in equivalents/liter (Eq/L). But the number of equivalents must be calculated based on the equivalent weight of the chemical reagent. So, the initial step of the calculation is to obtain the equivalent weight of Na2CO3.

The reactant is a salt, so its pEq is:

PM / (Sm x Vm)

The metal in Na2CO3 is Na. The subscript of Na (Sm) is 2 and its valence (Vm) is 1. Therefore, Sm x Vm equals 2.

pEq = PM / 2

= 106 g/mol ÷ 2 Eq/mol

= 53g/Eq

The Na2CO3 solution to be prepared is 2 N, so by definition it has a concentration of 2 Eq/L. Then, the concentration expressed in g/L can be found, by using the mathematical expression:

g/L = Eq/L (N) x pEq (g/Eq)

= 2 Eq/L x 53 g/Eq

= 106

So, to prepare 1 liter of a 2 N sodium carbonate solution, 106 g of the reagent are required.

Step 3

Weigh the calculated grams of the reagent on an analytical or precision balance, carefully so as not to make weighing errors.

Step 4

Dissolve the weighed reagent in a beaker and add an adequate volume of deionized or distilled water so that the volume in which the reagent dissolves does not exceed the stipulated volume.

step 5

Pour the contents of the beaker into a volumetric flask and add water to the mark. Finally, the volume of the reagent is transferred to a suitable container for its storage and use.

Examples of normal solutions

Example 1

How many grams of sodium hydroxide (NaOH) are required to prepare 1.5 liters of a 2N solution and what volume of 1N HCl is required to completely neutralize the NaOH? NaOH molecular weight = 40 g/mol.

Part A

The equivalent weight of NaOH is calculated as:

PEq NaOH = PM / No. OH

NaOH is a base that has only one OH.

pEq NaOH = 40 g/mol ÷ 1 Eq/mol

= 40g/Eq

The number of grams of NaOH needed to prepare a NaOH solution can be obtained by applying the relationship:

g/L of NaOH = Normality (Eq/L) x pEq (g/Eq)

= 2 Eq/L x 40 g/Eq

= 80g/L

Now, the grams of NaOH needed to prepare 1.5 L of a 2 N NaOH solution can be obtained:

g of NaOH = 80 g/L x 1.5 L

= 120g NaOH

part b

A characteristic of equivalents is that a number of them react with the same number of other equivalents.

The proposed reaction is a neutralization reaction, in which an acid (HCl) reacts with a base (NaOH) to produce a salt and water. Therefore, a number of equivalents of the acid (EqA) reacts with the same number of equivalents of a base (EqB) to produce its neutralization.

Knowing that the equivalents are related to normality and volume through the following expression:

Eq = V x N

The volume of HCl needed to neutralize the NaOH can be determined:

EqA = VA x NA

EqB = VB x NB

EqA = EqB

So,

VA x NA = VB x NB

We clear VA:

VA = VB x NB / NA

In this case, hydrochloric acid (1 N) and sodium hydroxide (2 N) intervene:

VA = (1.5 L x 2 Eq/L) / 1 Eq/L

= 3L

3 liters of a 1 N HCl solution are required to neutralize 1.5 liters of a 2 N NaOH solution.

Example 2

What will be the normality of a calcium chloride (CaCl2) solution that is prepared by dissolving 120 grams of the reagent in 1.5 liters? Molecular weight of CaCl2 = 111 g/mol

Let’s first determine the equivalent weight (pEq) of CaCl2. CaCl2 is a salt, therefore:

pEq = PM / (Sm x Vm)

The metal is calcium (Ca), its subscript is 1 (Sm) and its valence is 2 (Vm). Thus, we substitute:

pEq = 111 g/mol /(1 x 2)

= 55.5g/Eq

Finally, we proceed to determine the normality (Eq/L) of the solution. This calculation can be obtained by applying the appropriate conversion factors:

N = (120g/1.5L) x (Eq/55.5g)

= 1.44

Therefore, by definition the normality of the CaCl2 solution is 1.44 N

References

Ma. Berenice Charles Herrera. (September 18, 2011). Standard Solution (N). Recovered from: pdifresh.blogspot.com
Ankur Choudhary. (2020). Preparation of Molar and Normal Solutions. Retrieved from: pharmaguideline.com
CE Lab. (2020). What is a Normal Solution? Retrieved from: labce.com
Helmenstine, Anne Marie, Ph.D. (February 11, 2020). How to Calculate Normality (Chemistry). Retrieved from: thoughtco.com
Wikipedia. (2020). Equivalent concentration. Retrieved from: en.wikipedia.org

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