The pH Scale: Videos & Practice Problems

The pH scale, ranging from 0 to 14, measures acidity and basicity, with values below 7 indicating acidity and above 7 indicating basicity. The relationship between pH and hydroxide ion concentration (OH^-) is expressed as pH + pOH = 14. The concentration of hydrogen ions (H⁺) can be calculated using the equation H⁺ = 10^-pH, while OH^- = 10^-pOH. Understanding these concepts is crucial for determining the strength of acids and bases, which is essential for various chemical reactions and processes.

The pH Scale Concept 1

Understanding pH and pOH is essential in chemistry, particularly when dealing with acids and bases. The pH scale was developed to convert the typically small concentrations of hydrogen ions (H⁺) and hydroxide ions (OH^-) into more manageable numbers. Under standard conditions, the pH scale ranges from 0 to 14, but it is important to note that pH values can fall below 0 or exceed 14 when the concentration of acids or bases is significantly increased, such as when concentrations exceed 1 molar.

The relationship between pH, pOH, and ion concentrations is defined mathematically. The pH is calculated using the formula:

pH = -\log[H⁺]

Alternatively, since H⁺ and H₃O⁺ (hydronium ion) are equivalent, the formula can also be expressed as:

pH = -\log[H₃O⁺]

Similarly, pOH is determined by the concentration of hydroxide ions:

pOH = -\log[OH^-]

In summary, the pH and pOH scales provide a way to quantify the acidity or basicity of a solution, with the negative logarithm of the respective ion concentrations serving as the basis for these calculations. Understanding these concepts is crucial for predicting the behavior of chemical reactions in various environments.

The pH Scale Concept 2

Understanding the relationship between hydrogen ions (H⁺) and hydroxide ions (OH^-) is crucial for working with pH and pOH in chemistry. The pH scale is a logarithmic scale that measures the concentration of hydrogen ions in a solution, defined by the equation:

pH = -log[H⁺]

To manipulate this equation, we can multiply both sides by -1, resulting in:

-pH = log[H⁺]

Next, to isolate [H⁺], we exponentiate both sides using base 10, leading to the expression:

[H⁺] = 10^-pH

Similarly, we can derive the relationship for hydroxide ions (OH^-) using pOH. The pOH is defined as:

pOH = -log[OH^-]

By applying the same steps, we can express [OH^-] as:

[OH^-] = 10^-pOH

These relationships are essential for converting between pH and the concentrations of H⁺ and OH^-. For instance, if you are given a pH value, you can easily calculate the concentration of hydrogen ions using the formula [H⁺] = 10^-pH. Conversely, if you have a pOH value, you can find the concentration of hydroxide ions with [OH^-] = 10^-pOH. Mastering these conversions allows for a deeper understanding of acid-base chemistry and the behavior of solutions.

The pH Scale Concept 3

Understanding the pH scale is essential for classifying substances as acidic, basic, or neutral. A pH greater than 7 indicates a basic solution, where the concentration of hydroxide ions (OH^-) exceeds that of hydrogen ions (H⁺). In basic solutions, the stronger the base, the higher the pH, which correlates with an increased concentration of hydroxide ions. This relationship highlights that the strongest bases will have the highest pH values.

Conversely, a pH less than 7 signifies an acidic solution, characterized by a higher concentration of hydrogen ions compared to hydroxide ions. The strength of an acid inversely affects the pH; thus, stronger acids will exhibit lower pH values, indicating a greater concentration of hydrogen ions. A pH of exactly 7 is considered neutral, which occurs at 25 degrees Celsius. At this temperature, the ion product of water (K_w) is equal to 1.0 x 10^-14, meaning the concentrations of H⁺ and OH^- are equal.

It is crucial to recognize whether a substance is a weak or strong acid or base, as this knowledge is necessary for calculating pH using ICE (Initial, Change, Equilibrium) charts, especially when dealing with weak acids or bases. Remember that the neutral pH can vary with temperature changes, affecting the concentrations of H⁺ and OH^- ions accordingly.

The pH Scale Concept 4

The relationship between pH and pOH is fundamental in understanding acid-base chemistry. This relationship is expressed by the equation:

pH + pOH = 14

This equation stems from the ion product of water, represented as:

K_w = [H⁺][OH^-]

Here, K_w is the equilibrium constant for water, which is equal to 1.0 x 10^-14 at 25°C. To derive the pH and pOH from the concentrations of hydrogen ions [H⁺] and hydroxide ions [OH^-], we use the negative logarithm:

pH = -log[H⁺]

pOH = -log[OH^-]

When you take the negative log of the product of [H⁺] and [OH^-], it results in the addition of their respective pH and pOH values, reinforcing the connection between these two concepts. Thus, the equation pH + pOH = 14 is derived from the negative logarithm of K_w.

It's important to note that while the pH scale typically ranges from 0 to 14, extreme concentrations of acids or bases can result in pH values that fall outside this range.

The pH Scale Example 1

Understanding the relationship between pH, pOH, and ion concentrations is crucial in chemistry, particularly in aqueous solutions. The pH scale measures the acidity or basicity of a solution, with lower values indicating higher acidity. The pH of a solution is related to the concentration of hydrogen ions (H⁺) and hydroxide ions (OH^-).

To find the concentration of H⁺ ions from a given pH, the formula used is:

H⁺ = 10^-pH

For a solution with a pH of 6.12, the concentration of H⁺ ions can be calculated as:

H⁺ = 10^-6.12 = 7.59 x 10^-7 M.

Next, to find the concentration of hydroxide ions (OH^-), we can use the ion product constant of water (K_w), which at 25 degrees Celsius is:

K_w = [H⁺][OH^-] = 1.0 x 10^-14.

Rearranging this equation to solve for OH^- gives:

OH^- = K_w / [H⁺].

Substituting the known values:

OH^- = 1.0 x 10^-14 / 7.59 x 10^-7 = 1.32 x 10^-8 M.

Alternatively, we can find OH^- using pOH. The relationship between pH and pOH is given by:

pH + pOH = 14.

From the pH of 6.12, we can isolate pOH:

pOH = 14 - pH = 14 - 6.12 = 7.88.

Then, using the relationship between pOH and OH^-:

OH^- = 10^-pOH = 10^-7.88.

This method also yields the same concentration of hydroxide ions, confirming the consistency of these calculations.

In summary, both methods—calculating directly from H⁺ concentration or using pOH—are valid for determining the concentration of hydroxide ions in a solution. Choosing the method depends on personal preference, but it is essential to ensure proper use of parentheses in calculations to avoid errors.

Which of the following statements about aqueous solutions is/are true?

• A. For an basic solution the concentration of H₃O⁺ is greater than the concentration of OH^-.
• B. The pH of a neutral aqueous solution is 7.00 at all temperatures.
• C. An acidic solution under normal conditions has a pH value less than 7.00.
• D. If the concentration of H₃O⁺ decreases then the concentration of OH^- will also decrease.
• E. The pH of aqueous solutions is less than 7.

Answer: (C)

The pH Scale Example 2

In this discussion, we explore the calculations involving pH and pOH, particularly focusing on the concentration of hydroxide ions (OH^-) in a solution prepared by dissolving strontium hydroxide (Sr(OH)₂). When 0.235 moles of strontium hydroxide are dissolved in 750 mL of water, we first convert the volume from milliliters to liters, recognizing that 750 mL is equivalent to 0.750 liters.

To find the concentration of hydroxide ions, we need to determine the moles of OH^- produced. Each mole of strontium hydroxide yields two moles of hydroxide ions due to its chemical formula. Therefore, from 0.235 moles of strontium hydroxide, we calculate:

0.235 moles Sr(OH)₂ × 2 moles OH^- / 1 mole Sr(OH)₂ = 0.470 moles OH^-

Next, we can find the molarity (M) of hydroxide ions using the formula for molarity, which is defined as:

Molarity (M) = moles of solute / liters of solution

Substituting the values we have:

M = 0.470 moles OH^- / 0.750 liters = 0.627 M

For the second part of the problem, we need to find the concentration of hydrogen ions (H⁺). The relationship between hydroxide and hydrogen ion concentrations is governed by the ion product of water (K_w), which is defined as:

K_w = [H⁺][OH^-]

At 25°C, K_w is equal to 1.0 × 10^-14. Given the concentration of hydroxide ions, we can rearrange the equation to solve for [H⁺]:

[H⁺] = K_w / [OH^-]

Substituting the known values:

[H⁺] = 1.0 × 10^-14 / 0.627 = 1.59 × 10^-14

This calculation illustrates the stoichiometric relationships and unit conversions necessary to determine the concentrations of hydroxide and hydrogen ions in a solution. Understanding these relationships is crucial for solving similar problems in acid-base chemistry.

What is the Kw of pure water at 20.0°C, if the pH is 7.083?

• A. 8.26 × 10^-8
• B. 6.82 × 10^-15
• C. 7.23 × 10^-14
• D. 1.00 × 10^-14

Answer: (B)

Here’s what students ask on this topic:

What is the pH scale and how does it work?

The pH scale measures the acidity or basicity of a solution, ranging from 0 to 14. A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic solution. A pH of 7 is considered neutral. The scale is logarithmic, meaning each whole number change represents a tenfold change in hydrogen ion (H⁺) concentration. The pH is calculated using the formula:

$\mathrm{pH}=-\log \left(H_{+}\right)$

This makes it easier to handle the very small concentrations of H⁺ ions typically found in solutions.

How do you calculate the pH of a solution?

To calculate the pH of a solution, you need to know the concentration of hydrogen ions (H⁺) in moles per liter (M). The pH is then calculated using the formula:

$\mathrm{pH}=-\log \left(H_{+}\right)$

For example, if the H⁺ concentration is 1.0 × 10^-3 M, the pH would be:

$\mathrm{pH}=-\log (1.0\times {10}^{-3})=3$

What is the relationship between pH and pOH?

The relationship between pH and pOH is given by the equation:

$\mathrm{pH}+\mathrm{pOH}=14$

This equation arises from the ion product of water (K_w), which is 1.0 × 10^-14 at 25°C. Since:

$K_w=H_{+}\times {\mathrm{OH}}_{-}$

Taking the negative logarithm of both sides gives:

$-\log \left(K_w\right)=-\log \left(H_{+}\right)-\log \left({\mathrm{OH}}_{-}\right)$

Which simplifies to:

$\mathrm{pH}+\mathrm{pOH}=14$

How do you determine if a solution is acidic, basic, or neutral?

A solution's pH determines whether it is acidic, basic, or neutral. If the pH is less than 7, the solution is acidic, meaning it has a higher concentration of hydrogen ions (H⁺) than hydroxide ions (OH^-). If the pH is greater than 7, the solution is basic, meaning it has a higher concentration of OH^- ions than H⁺ ions. A pH of 7 indicates a neutral solution, where the concentrations of H⁺ and OH^- are equal. This is typically true at 25°C, where the ion product of water (K_w) is 1.0 × 10^-14.

What is the significance of the pH scale in chemistry?

The pH scale is crucial in chemistry because it helps quantify the acidity or basicity of a solution, which is essential for understanding chemical reactions and processes. Many biological and chemical systems are highly sensitive to pH changes. For example, enzyme activity in biological systems often depends on maintaining a specific pH range. In industrial processes, pH control is vital for product quality and safety. The pH scale also helps in determining the strength of acids and bases, which is important for titrations and other analytical techniques.