Question

In a certain experiment, a student finds that the \(\mathrm{pHs}\) of \(0.10-M\) solutions of three potassium salts \(\mathrm{KX}, \mathrm{KY},\) and \(\mathrm{KZ}\) are 7.0,9.0 , and 11.0 , respectively. Arrange the acids HX, HY, and HZ in order of increasing acid strength.

Short answer

The acids are ranked as HZ < HY < HX in increasing acid strength.

Step-by-step solution

Understanding pH and Acidity

The pH of a solution indicates its acidity or basicity. A pH less than 7 is acidic, a pH of 7 is neutral, and a pH greater than 7 is basic. Strong acids will dissociate completely in water, resulting in a lower pH compared to weak acids.

Analyzing Given pH Values

The problem states that the pH values of 0.10-M solutions of salts are: KX is 7.0, KY is 9.0, and KZ is 11.0. This implies that HX is neutral, HY is weakly basic, and HZ is more basic.

Relating pH to Acid Strength

A higher pH (more basic) corresponding to a salt means the conjugate acid is weaker. Therefore, the conjugate acids HX, HY, and HZ have their strengths inversely related to the measured pH values: lower pH values indicate stronger acids.

Ordering Acids by Strength

Given the pH values: 7.0 (HX), 9.0 (HY), 11.0 (HZ), the order of increasing acid strength of HX, HY, and HZ is HZ < HY < HX. As pH increases (more basic), acid strength decreases.

Key concepts

pH and Acidity

The pH scale is a numerical representation of how acidic or basic a solution is. It ranges from 0 to 14. Typically, solutions with a pH less than 7 are considered acidic, while those with a pH greater than 7 are basic. A neutral solution, like pure water, has a pH of exactly 7. The precise level on the pH scale helps determine the strength of an acid or base in a solution.
A strong acid will completely donate its protons (hydrogen ions) in water, leading to a lower pH. This means if we have two solutions and one is more acidic, its pH will be lower. On the other hand, a strong base will accept more protons and have a higher pH. Thus, when comparing acid strengths, the lower the pH, the stronger the acid. This is crucial in understanding how the acidity of a solution conveys the concentration of hydrogen ions it contains.

Conjugate Acid

Conjugate acids are formed when a base gains a hydrogen ion (proton). To understand this, consider an acid-base reaction: an acid loses a proton to become a conjugate base, while the base gains a proton to become a conjugate acid. This interplay forms the foundation of acid-base chemistry.
In simpler terms, a conjugate acid is the particle that results when a base accepts a proton. For instance, if you take the ammonium ion (NH₄⁺), it is the conjugate acid of the base ammonia (NH₃). The strength of a conjugate acid is inversely related to the strength of the parent base. If the base is very strong and holds its protons tightly, its conjugate acid is weak and does not give them up easily. This principle helps to predict how substances will behave in various chemical reactions and is essential for ordering the strength of acids based on their conjugate relationships.

Dissociation in Water

Dissociation describes when a compound splits into smaller particles, often ions, especially when in an aqueous (water-based) solution. For acids, this means splitting into hydrogen ions ( H^+ ) and anions. The extent to which an acid dissociates in water is a key indicator of its strength.
Strong acids dissociate completely, meaning they release a large number of H^+ ions into the solution. This increases the concentration of hydrogen ions, lowering the pH and indicating higher acidity. Conversely, weak acids only partially dissociate, releasing fewer hydrogen ions and resulting in higher pH values. Understanding this concept is vital for interpreting how acid strength relates to pH and how different substances will behave when mixed with water.