Question

A certain salt, MX (containing the \(\mathrm{M}^{+}\) and \(\mathrm{X}^{-}\) ions), is dissolved in water, and the \(\mathrm{pH}\) of the resulting solution is 7.0. Can you say anything about the strengths of the acid and the base from which the salt is derived?

Short answer

The given salt MX is derived from a strong acid and a strong base since its solution in water is neutral (pH = 7). This is because a strong acid and strong base neutralize each other's effects, leading to a neutral pH. Thus, neither the \(\mathrm{M}^{+}\) cation nor \(\mathrm{X}^{-}\) anion exhibits acidic or basic behaviour.

Step-by-step solution

Understand pH and its relation to acids and bases

The pH scale is used to determine how acidic or alkaline a water-based solution is. Acidic solutions have lower pH values (0-6), basic solutions have higher pH values (8-14), and 7 is neutral. The given pH value is 7.0, implying the solution is neutral. Therefore, the solution doesn't favor acidity or basicity.

Becoming familiar with the relationship between salts and their parent acid and base

Acid-base neutralization forms a salt. If a salt is formed by neutralizing a strong acid with a strong base, it doesn't affect the pH of the solution when dissolved in water as it’s neutral. If a salt is produced by a strong acid and weak base, the solution will be acidic when dissolved. On the other hand, if it's from a weak acid and strong base, the resulting solution will be basic.

Identify the strengths of the acid and base

As the solution of the salt MX is neutral (pH 7.0), it implies the salt is derived from the neutralization of a strong acid by a strong base. The reasoning is that a strong acid and a strong base neutralize each other's effects leading to a neutral solution. Neither the \(\mathrm{M}^{+}\) cation nor \(\mathrm{X}^{-}\) anion acts as an acid or base in water, that's why the solution is not acidic or basic.

Key concepts

pH Scale

The pH scale is like a ruler for measuring how acidic or basic a solution is. It ranges from 0 to 14, where each step represents a tenfold change in acidity or basicity. A solution with a pH less than 7 is considered acidic. This is because it has a higher concentration of hydrogen ions (\( ext{H}^+ \)) compared to hydroxide ions (\( ext{OH}^- \)).
On the other hand, basic solutions have a pH greater than 7, meaning they have more hydroxide ions than hydrogen ions.
When a solution has a pH of exactly 7, like pure water, it is neutral. In this case, the concentrations of hydrogen and hydroxide ions are equal.

Understanding the pH of a solution gives us insights into its chemical nature, which is essential for predicting reactions with other substances.

• Acidic: pH < 7
• Neutral: pH = 7
• Basic: pH > 7

This scale is fundamental in the study of chemistry, especially in understanding how different substances interact in processes like neutralization.

Acid-Base Theory

The acid-base theory is a cornerstone in chemistry that helps us understand how substances behave in water. According to the theory, **acids** are substances that donate protons (\( ext{H}^+ \)), and **bases** are those that accept protons.
When an acid reacts with a base, they form water and a salt. This reaction is called neutralization. What's fascinating is that the properties of the resulting salt can tell us a lot about the strengths of the original acid and base.

In the creation of salts:

• A strong acid neutralized by a strong base results in a neutral salt, which does not change the pH when dissolved in water.
• A strong acid neutralized by a weak base forms an acidic salt, slightly lowering the pH of the solution.
• A weak acid neutralized by a strong base forms a basic salt, slightly increasing the pH of the solution.

Thus, the acid-base theory not only explains the formation of these salts but also their effect on the pH of solutions.

Strong Acids and Bases

Strong acids and bases are special because they completely dissociate into their ions when dissolved in water. This complete dissociation is what makes them strong. For instance, hydrochloric acid (\( ext{HCl} \)) splits into hydrogen ions and chloride ions in water, while sodium hydroxide (\( ext{NaOH} \)) separates into sodium ions and hydroxide ions.
When a strong acid interacts with a strong base, they neutralize each other perfectly because both dissociate fully:

• This results in a neutral solution with a pH of 7 because the \( ext{H}^+ \) ions from the acid and the \( ext{OH}^- \) ions from the base combine to form water.

Only strong acids and strong bases can consistently create a neutral salt in this fashion because their full dissociation ensures that no residual acidic or basic ions remain in the solution. Thus, understanding the behavior of strong acids and bases is essential in predicting the outcomes of neutralization reactions.