Question

Detemine the pOH of each solution and classify it as acidic, basic, or neutral. (a) \(\left[\mathrm{OH}^{-}\right]=1.5 \times 10^{-9} \mathrm{M}\) (b) \(\left[\mathrm{OH}^{-}\right]=7.0 \times 10^{-5} \mathrm{M}\) (c) \(\left[\mathrm{OH}^{-}\right]=1.0 \times 10^{-7} \mathrm{M}\) (d) \(\left[\mathrm{OH}^{-}\right]=8.8 \times 10^{-3} \mathrm{M}\)

Short answer

(a) pOH \textgreater 7, acidic; (b) pOH \textless 7, basic; (c) pOH = 7, neutral; (d) pOH \textless 7, basic.

Step-by-step solution

Understanding pOH

pOH is a measure of hydroxide ion concentration and is calculated using the formula pOH = -log[OH-]. A pOH less than 7 indicates a basic solution, pOH greater than 7 indicates an acidic solution, and a pOH of 7 indicates a neutral solution.

Calculating pOH for (a)

For the hydroxide ion concentration of 1.5 x 10^-9 M, pOH = -log(1.5 x 10^-9).

Classifying the solution for (a)

After calculating the pOH, if it's greater than 7 the solution is acidic.

Calculating pOH for (b)

For the hydroxide ion concentration of 7.0 x 10^-5 M, pOH = -log(7.0 x 10^-5).

Classifying the solution for (b)

After calculating the pOH, if it's less than 7 the solution is basic.

Calculating pOH for (c)

For the hydroxide ion concentration of 1.0 x 10^-7 M, pOH = -log(1.0 x 10^-7).

Classifying the solution for (c)

After calculating the pOH and finding it to be 7, the solution is neutral.

Calculating pOH for (d)

For the hydroxide ion concentration of 8.8 x 10^-3 M, pOH = -log(8.8 x 10^-3).

Classifying the solution for (d)

After calculating the pOH, if it's less than 7, the solution is basic.

Key concepts

Hydroxide Ion Concentration

When studying solutions, the hydroxide ion concentration is a crucial concept, especially in chemistry. It dictates the overall behavior of a solution: whether it will be acidic or basic. A higher concentration of hydroxide ions \(\left[\mathrm{OH}^{-}\right]\) indicates a basic solution, while a lower concentration points to an acidic environment.

For example, take solution (a) \(\left[\mathrm{OH}^{-}\right]=1.5 \times 10^{-9} \mathrm{M}\). Its hydroxide ion concentration is quite low, which typically would lead us to think the solution is acidic. However, to determine its actual nature, we calculate its pOH and then use that value to infer its acidity or basicity.

To improve clarity in exercises like this, it's imperative to highlight the inverse relationship between the hydroxide ion concentration and pOH. When \(\left[\mathrm{OH}^{-}\right]\) increases, pOH decreases, indicating a more basic solution, and vice versa for acidic solutions.

Acidic and Basic Solutions

Acidic and basic solutions are differentiated by their pH and pOH values. For a solution to be classified as acidic, its pOH must be greater than 7, while a basic solution will have a pOH less than 7. A pOH of exactly 7 is characteristic of a neutral solution, such as pure water at room temperature.

Looking at solution (b) \(\left[\mathrm{OH}^{-}\right]=7.0 \times 10^{-5} \mathrm{M}\), applying the pOH calculation shows us it has a lower pOH, thus classifying it as basic. Conversely, a solution like (a) would typically be considered acidic due to its higher pOH value obtained from the calculation.

To assist students better, it would be beneficial to stress that the dividing line between acidic and basic solutions is pOH 7. Moreover, contextual examples or real-world applications could help cement the understanding of these concepts.

pH and pOH Relationship

Understanding the relationship between pH and pOH is vital for mastering the basics of acid-base chemistry. The pH measures the hydrogen ion concentration and ranges from 0 to 14, just like the pOH scale. The sum of pH and pOH always equals 14, a constant derived from the ion product of water at room temperature \(K_{w} = 10^{-14}\).

For solution (c) \(\left[\mathrm{OH}^{-}\right]=1.0 \times 10^{-7} \mathrm{M}\), the pOH equals 7, indicating a neutral solution. In this scenario, the pH would also be 7, satisfying the relationship pH + pOH = 14. To help learners grasp this concept better, it's essential to reinforce that pH and pOH are two sides of the same coin - one measures acidity while the other measures basicity but both are connected through the neutral point of water.

Utilizing diagrams or visual aids showing the pH scale with corresponding pOH values could significantly enhance comprehension and retention of this concept.