Saline solutions of strong acids and strong bases/weak bases and strong acids

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Hello friends!

In daily life salts are widely used, its use ranges from adding them to food as NaCl or common salt, as a food preservative, as an ingredient in bleaches such as NaOCl, in the chemical industry in the manufacture of soaps, paints, paper, water treatment and others.

Each of these salts has characteristics that distinguish them. Among the types of salts, we can find neutral, acidic or basic salts, so it is important to study their properties, especially to know the pH of the solutions when they are dissolved. Next we will focus on the calculation of pH of neutral and acid salts, I invite you to read!

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Source:@yusvelasquez

In chemistry, a salt is defined as a compound containing a cation that comes from a base and an anion that comes from an acid, which are formed when a neutralization reaction is carried out, which in general form is written as:

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Based on the theory of Bronsted and Lowry, which states that an ion can act as an acid or as a base, it can be deduced that the pH of a salt solution will depend on the acidity or basicity of its corresponding ions and their tendency to hydrolyze, which will determine whether the pH is neutral, acidic or basic.

It is important to begin by focusing on the hydrolysis reaction, which occurs between a substance and water or its ions. The union of the anion of a weak acid with H3O+ of water can occur to form molecules of the non-ionized acid, when H3O+ is eliminated, the water equilibrium is broken and basic solutions are produced.

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  • Let us begin by studying salts that come from strong acids and strong soluble bases:
    An example of this type of salt is sodium chloride, NaCl also known as common salt, which is formed by the following reaction:

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Sodium chloride. Source:@yusvelasquez

Sodium chloride is a white solid that in some cases is used for the preservation of meats, it is also used medicinally to hydrate people and provide them with electrolytes, and in different industries it is used for the production of products such as chlorine, detergents, bleaches, among others.

The dissociation reaction is given by:

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Source:@yusvelasquez

If we analyze this reaction, we can observe that both Na+ and Cl- ions come from strong species, so they are weak acids and bases and therefore have no tendency to react with water, so they have no significant effect on the pH of the solution.

Calculation of pH

In this case the pH of the solution depends on the contribution of H3O+ from the water and is expected to be neutral. Thus it follows that,

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Some examples can also be NaBr, NaI, KNO3 among others.

  • In the second case we have salts from strong acids and weak bases :

In this category we can cite ammonium chloride, NH4Cl. Ammonium chloride, as shown in the figure, is a white, highly water-soluble salt, widely used in fertilizer production, dry cell production, water treatment and as a food additive.

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Ammonium Chloride. Source: wikimedia.org

Let us now analyze its influence on the pH of the solution when dissolved in water.

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Source:@yusvelasquez

As we can see, the NH4+ cation comes from a weak base and therefore has a strong tendency to hydrolyze and the pH of the solution depends on this, since the Cl- ion comes from a strong acid as described in the previous case and does not hydrolyze.

The hydrolysis reaction is then given by

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Calculation of pH

To calculate the pH of salts of any type except neutral salts, we can use the equilibrium table, as shown in the example below.

Suppose we want to calculate the pH of a 0.20 mol/L ammonium chloride solution.

We can start by writing down the hydrolysis reaction and complete the equilibrium table

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We express the equilibrium constant (Ka) and calculate the value of the conjugate acid constant starting from the value of the base from which this acid was formed. In this case we will use 1.8*10-5 which corresponds to the ammonia constant according to the bibliography.

We substitute the values

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and find the value of X which corresponds to the concentration of H3O+.

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We calculate the pH with the H3O+ concentration.

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As we see the resulting pH of 4.98 is < 7.0.

We can conclude then, that acidic cations coming from weak bases produce acidic solutions.

Among other examples are NH4NO3, NH4Br, NH4NO3 among others.

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Source: @yusvelasquez

Well friends for the moment I say goodbye, I hope the information presented is very useful, thanks for reading, until next time!


References

  • Whitten K. y Gailey, K. (1985). Química General. México: Nueva Editoral Interamericana
  • Atkins, P. y Jones, L. (2006). Principios de Química. Los caminos del descubrimiento. Buenos Aires: Médica Panamericana.
  • Cloruro de sodio. Disponible en chemicalsafetyfacts.org
  • Cloruro de amonio.Wikipedia.com


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2 comments
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I remember my chemistry classes at university! This post is very important to understand the behavior of acids and why we need to respect them! We deal with acids in our kitchen or in other daily tasks, Thanks for the post
!1UP

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