Question

What is the \(\mathrm{pH}\) of a neutral aqueous solution? How many times as acidic is a solution of \(\mathrm{pH} 2\) than one of \(\mathrm{pH} 6 ?\)

Short answer

A neutral solution has a pH of 7. A solution with pH 2 is 10,000 times more acidic than one with pH 6.

Step-by-step solution

Understanding pH of Neutral Solution

In a neutral aqueous solution, the concentrations of hydrogen ions \( (H^+) \) and hydroxide ions \( (OH^- )\) are equal. At \(25^\circ C \), the \(H^+ \) concentration of a neutral solution is \( 1 \times 10^{-7} \) M, leading to a \( \text{pH} \) of 7, derived from the formula \( \text{pH} = -\log[H^+] \).

Calculating the Difference in pH

Given \( \text{pH} \) values of 6 and 2, the task is to understand how these relate in terms of acidity. \( \text{pH} \) is a logarithmic scale, which means a difference of \( 1 \) unit indicates a tenfold difference in \(H^+\) concentration.

Calculating Acidicity Comparison

To find how many times more acidic the solution with \( \text{pH} \) of 2 is compared to one with \( \text{pH} \) of 6, calculate the difference in pH units: \( 6 - 2 = 4 \). Since each unit represents a tenfold difference, \(10^4 = 10,000 \). This means the solution with \( \text{pH} \) of 2 is 10,000 times more acidic than one with \( \text{pH} \) of 6.

Key concepts

Aqueous Solution

An aqueous solution is a mixture where water acts as the solvent. In everyday terms, it's a solution in which other substances are dissolved in water. The term "aqueous" comes from "aqua," meaning water in Latin. These solutions are very common in chemistry, mainly because water is known as a "universal solvent."
🔎 Why is it important? Aqueous solutions are important because they allow substances to interact in a liquid medium, which is crucial for many chemical reactions.

• Water can dissolve a variety of substances including salts, acids and bases.
• The properties of substances can change when dissolved in water due to interactions with water molecules.

Understanding aqueous solutions helps in studying various chemical reactions, as well as biological processes that occur in living organisms.

Acidity

Acidity refers to the concentration of hydrogen ions \( \left(H^+\right) \) in a solution. It's a measure of how acidic a solution is. The more hydrogen ions present, the more acidic the solution. Acidity is often measured using the pH scale, which ranges from 0 to 14.
🔎 How is acidity measured? The pH value helps determine the acidity level of a solution:

• < 7 indicates an acidic solution where more \(H^+\) ions are present.
• 7 indicates a neutral solution, typical for pure water.
• > 7 indicates a basic or alkaline solution where fewer \(H^+\) ions and more \(OH^-\) ions exist.

Understanding acidity is essential for various applications, such as in chemical reactions, agriculture, medicine, and food science.

Hydrogen Ions

Hydrogen ions \( \left(H^+\right) \) play a critical role in determining the acidity of a solution. They are simply hydrogen atoms that have lost an electron, leaving behind a positively charged particle. This positive charge is what contributes to the solution's acidic nature.
🔎 Why are hydrogen ions important? Their concentration in a solution defines its pH level, affecting the solution's properties.

• Higher concentrations of \(H^+\) ions mean a lower pH and a more acidic solution.
• Hydrogen ions are involved in important biochemical processes, such as cellular respiration and photosynthesis.

By understanding hydrogen ion concentration, scientists can predict and control the behavior of chemical reactions and biological systems.

Neutral Solution

A neutral solution is one where the concentration of hydrogen ions \( \left(H^+\right) \) and hydroxide ions \( \left(OH^-\right) \) is equal. At 25°C, this balance results in a pH of 7, which is considered neutral. Pure water is a perfect example of a neutral solution.
🔎 Key points about neutral solutions:

• In a neutral solution, \(H^+\) equals \(OH^-\), ensuring no acidic or basic dominance.
• The neutral pH is critical for many biological processes, including digestion and enzyme function.
• A neutral pH ensures stability in chemical reactions where neither acidic nor basic conditions are desired.

Understanding neutral solutions helps in identifying how far a particular solution deviates from neutrality and what adjustments, if any, might be needed.