Question

Calculate the \(\mathrm{pOH}\) value corresponding to each of the \(\mathrm{pH}\) values listed, and tell whether each solution is acidic or basic. a. \(\mathrm{pH}=10.75\) b. \(\mathrm{pH}=3.66\) c. \(\mathrm{pH}=1.98\) d. \(\mathrm{pH}=12.47\)

Short answer

a. \(\mathrm{pOH}=3.25\), basic solution b. \(\mathrm{pOH}=10.34\), acidic solution c. \(\mathrm{pOH}=12.02\), acidic solution d. \(\mathrm{pOH}=1.53\), basic solution

Step-by-step solution

Determine the pOH values

We know that \(\mathrm{pH} + \mathrm{pOH} = 14\). Thus, we can calculate the pOH values by subtracting the given pH values from 14. a. \(\mathrm{pOH} = 14 - 10.75 = 3.25\) b. \(\mathrm{pOH} = 14 - 3.66 = 10.34\) c. \(\mathrm{pOH} = 14 - 1.98 = 12.02\) d. \(\mathrm{pOH} = 14 - 12.47 = 1.53\)

Determine if the solution is acidic or basic

Next, we will determine whether each solution is acidic or basic based on its pH value. a. \(\mathrm{pH}=10.75\): Since the pH value is greater than 7, this solution is basic. b. \(\mathrm{pH}=3.66\): Since the pH value is less than 7, this solution is acidic. c. \(\mathrm{pH}=1.98\): Since the pH value is less than 7, this solution is acidic. d. \(\mathrm{pH}=12.47\): Since the pH value is greater than 7, this solution is basic.

Final Results

a. \(\mathrm{pOH}=3.25\), basic solution b. \(\mathrm{pOH}=10.34\), acidic solution c. \(\mathrm{pOH}=12.02\), acidic solution d. \(\mathrm{pOH}=1.53\), basic solution

Key concepts

Acid-Base Chemistry

Understanding acid-base chemistry is crucial for comprehending how substances interact in aqueous solutions. Acids and bases are two fundamental categories of compounds that have unique characteristics. Acids are substances that can donate protons (hydrogen ions, H+) when dissolved in water, which increases the concentration of H+ ions in the solution. Common examples include hydrochloric acid (HCl) and citric acid. On the other hand, bases are substances that can accept protons or donate hydroxide ions (OH-), decreasing the concentration of H+ ions in the solution. Sodium hydroxide (NaOH) is a well-known base.

Acid-base reactions are often described by the Bronsted-Lowry theory, where acids are proton donors, and bases are proton acceptors. The strengths of acids and bases depend on their ability to donate or accept protons, which is quantified by their dissociation constants (Ka and Kb respectively). A strong acid or base dissociates completely in water, while a weak one does not.

The concept of pH and pOH are integral parts of acid-base chemistry. They give us a quantitative measure of the acidity or basicity of a solution by relating to the concentration of H+ and OH- ions, respectively. When solving problems in acid-base chemistry, we often use these measures to understand the nature of the solution and predict the outcomes of reactions.

pH Scale

The pH scale is an invaluable tool used to measure the acidity or basicity of a solution. It ranges typically from 0 to 14, with lower values indicating acidic characteristics and higher values denoting basic or alkaline properties. A pH value of 7 is considered neutral, which is the pH level of pure water at 25°C.

The term 'pH' stands for 'potential of hydrogen' or 'power of hydrogen' and represents the negative logarithm (base 10) of the concentration of hydrogen ions (pH=-log_{10} [H+]). In an aqueous solution with a pH of 7, the concentration of H+ ions is exactly equal to 1 x 10^-7 moles per liter. The pH formula shows that as H+ ion concentration increases (more acidic), the pH value decreases, and as H+ ion concentration decreases (more basic), the pH value increases.

Understanding the pH scale is essential when performing pH and pOH calculations as part of acid-base problems. It allows us to conclude the nature of the solution, just like it was presented in the original exercise's step-by-step solution.

Acidity and Basicity

Acidity and basicity describe the strength of an acid or base in a solution. The term acidity is associated with the presence of excess hydrogen ions (H+) in a solution, making it acidic, while basicity refers to the presence of excess hydroxide ions (OH-), making it basic or alkaline. Establishing whether a solution is acidic or basic is often a fundamental aspect of chemistry exercises and practical experiments.

As explained in the pH and pOH calculations from the original exercises, the pH value is a direct indicator of acidity—lower pH means higher acidity—and the pOH value is a direct indicator of basicity—lower pOH means higher basicity. When the pH is less than 7, the solution is acidic; when it is more than 7, it is basic. The scenario is reversed for pOH, where more than 7 indicates acidity, and less than 7 indicates basicity.

The relationship between pH and pOH is also vital; it is expressed by the equation pH + pOH = 14, which is derived from the product of H+ and OH- ion concentration in water at 25°C ([H+] n[OH-] = 1 x 10^-14). This equation not only helps us to find one value when the other is known but also aids in reinforcing the concept of neutrality, where pH and pOH are equal at the value of 7. This relationship was utilized in solving the exercise to determine the nature of solutions as either acidic or basic.