pH of a Weak Base

Khan Academy
7 Sept 200913:00
EducationalLearning
32 Likes 10 Comments

TLDRThe video script discusses the concept of a weak base, using ammonia as an example. It explains that ammonia acts as a weak base by accepting a proton from water, forming ammonium and increasing hydroxide concentration. The script then delves into the equilibrium reaction, utilizing the Bronsted-Lowry definition and calculating the equilibrium constant (Kb) from a provided pKb value. Through mathematical calculations, the script determines the concentration of hydroxide ions and subsequently the pOH and pH of the solution, concluding that the pH is greater than 7, which is consistent with ammonia being a weak base.

Takeaways
  • 📚 The script discusses the concept of a weak base, using ammonia (NH3) as an example.
  • 🧪 Ammonia is a weak base because it accepts a proton from water, resulting in an increase in hydroxide (OH-) concentration.
  • 🔄 The reaction between ammonia and water is an equilibrium reaction, not going in just one direction.
  • 📈 The Bronsted-Lowry definition is highlighted, where a base is a proton acceptor, which is the case for ammonia.
  • 🌡️ The pH of a solution with 0.2 molar of NH3 is to be determined using the equilibrium constant (Kb).
  • 🔢 The pKb value for ammonia is given as 4.75, which is used to calculate the equilibrium constant Kb = 10^(-4.75).
  • 💡 The calculation involves setting up an equation using the equilibrium constant and solving for the hydroxide concentration [OH-].
  • 📌 A quadratic equation is used to solve for the unknown concentration, with the consideration that the concentration cannot be negative.
  • 🧮 The hydroxide concentration is found to be approximately 1.8 x 10^(-5) M, leading to a pOH of 2.72.
  • 🔄 The relationship between pOH and pH is used to find the pH of the solution, where pH = 14 - pOH.
  • 📈 The final pH of the solution with 0.2 molar NH3 is calculated to be 11.3, indicating the basic nature of ammonia.
Q & A

  • What is a weak base and how does it interact with water?

    -A weak base, such as ammonia, is a substance that partially accepts hydrogen ions (protons) from water, leading to the formation of hydroxide ions (OH-) and increasing the hydroxide concentration in the solution. This interaction is reversible, forming an equilibrium reaction.

  • How does the Bronsted-Lowry definition apply to a weak base like ammonia?

    -According to the Bronsted-Lowry definition, a weak base is a proton acceptor. Ammonia accepts a proton from water, forming ammonium (NH4+), and in doing so, it acts as a base by increasing the concentration of hydroxide ions in the solution.

  • What is the equilibrium constant (Kb) for the base reaction involving ammonia, and how was it determined?

    -The equilibrium constant (Kb) for the base reaction involving ammonia is 1.8 x 10^-5. This value was determined by using the pKb value of 4.75 found on Wikipedia and calculating the antilog (10^-4.75) to obtain the Kb value.

  • How does the concentration of hydroxide ions (OH-) relate to the concentration of hydrogen ions (H+) in an aqueous solution?

    -In an aqueous solution, the sum of the concentrations of hydroxide ions (OH-) and hydrogen ions (H+) must equal 10^-14 at 25°C, which is the ion product of water. This relationship is used to calculate the pH and pOH of the solution.

  • What is the process for calculating the pH of a solution with a weak base like ammonia?

    -To calculate the pH of a solution with a weak base, one must first determine the equilibrium constant (Kb) for the base reaction, then set up an equation to solve for the hydroxide ion concentration (OH-). Once the hydroxide concentration is found, the pOH is calculated using the negative logarithm of this concentration. Finally, the pH is determined by subtracting the pOH from 14 at 25°C.

  • What is the pKb value for ammonia, and how does it relate to the Kb value?

    -The pKb value for ammonia is 4.75. This value represents the negative logarithm (base 10) of the equilibrium constant (Kb) for the base reaction. To find the Kb value, one calculates the antilog (10^-pKb), which in this case is 10^-4.75, resulting in a Kb value of 1.8 x 10^-5.

  • What is the role of the weak base ammonia in an aqueous solution?

    -In an aqueous solution, the weak base ammonia (NH3) reacts with water to form ammonium (NH4+) and hydroxide ions (OH-), increasing the hydroxide concentration and thus raising the pH of the solution above 7, indicating its basic nature.

  • How does the concentration of hydroxide ions (OH-) influence the pH of a solution?

    -The concentration of hydroxide ions (OH-) is inversely related to the pH of a solution. A higher concentration of hydroxide ions corresponds to a lower pOH and, consequently, a higher pH, indicating a more basic solution.

  • What is the relationship between pKb and the equilibrium reaction of a weak base?

    -The pKb value is a measure of the strength of a weak base in an aqueous solution. It represents the negative logarithm of the base's equilibrium constant (Kb). A lower pKb value indicates a stronger base, as it means a larger Kb value and a greater extent of the base's reaction with water.

  • How does the concentration of ammonia (NH3) affect the pH of the solution?

    -The concentration of ammonia (NH3) directly influences the pH of the solution. A higher concentration of NH3 will result in a greater amount of NH3 reacting with water to form hydroxide ions (OH-), leading to a higher pH value, indicating a more basic solution.

  • What is the significance of the equilibrium constant (Kb) in the context of a weak base?

    -The equilibrium constant (Kb) for a weak base indicates the extent to which the base will react with water to form its conjugate acid and hydroxide ions. A larger Kb value signifies a stronger base, as it will drive the equilibrium reaction further towards the formation of the conjugate acid and hydroxide ions.

  • What is the mathematical process involved in finding the hydroxide ion concentration in a weak base solution?

    -The mathematical process involves setting up an equation based on the equilibrium constant (Kb) and solving for the unknown concentration of hydroxide ions (OH-). This typically involves a quadratic equation where the concentration of reactants and products is used to isolate and solve for the variable of interest, which in this case is the hydroxide ion concentration.

  • How does the concentration of hydrogen ions (H+) change when a weak base like ammonia is added to water?

    -When a weak base like ammonia is added to water, it accepts a proton from water, resulting in the formation of hydroxide ions (OH-) and ammonium ions (NH4+). This process decreases the concentration of hydrogen ions (H+), leading to an increase in pH and making the solution more basic.

Outlines
00:00
📚 Introduction to Weak Bases: Ammonia

This paragraph introduces the concept of weak bases using ammonia (NH3) as an example. It explains that ammonia is a weak base because it accepts a proton from water, resulting in an increase in hydroxide concentration. The equilibrium nature of the reaction is highlighted, along with the Bronsted-Lowry definition of a base as a proton acceptor. The paragraph sets up the problem of calculating the pH of a 0.2 molar NH3 solution and outlines the first steps in finding the equilibrium constant (Kb) for the base reaction by referring to Wikipedia. The calculation of Kb as 10^-4.75 is presented, and the process of solving for the hydroxide concentration is initiated.

05:00
🧪 Mathematical Setup for pH Calculation

This paragraph delves into the mathematical aspect of calculating the pH of the ammonia solution. It describes the process of setting up a quadratic equation to solve for the unknown variable 'x', which represents the concentration of hydroxide ions (OH-) and ammonium ions (NH4+). The paragraph explains the steps of multiplying the equation and rearranging it to solve for 'x', including the use of heuristics to simplify the equation when certain terms are significantly smaller than others. The calculation continues with the determination of the values for a, b, and c in the quadratic equation and the use of these values to find the square root and ultimately 'x'.

10:04
📈 Determining pOH and pH of the Ammonia Solution

The final paragraph focuses on calculating the pOH and subsequently the pH of the ammonia solution. It explains the relationship between the concentration of hydroxide ions found in the previous paragraph and the pOH, as well as the relationship between pOH and pH at 25 degrees Celsius. The paragraph concludes with the calculation of the pH by subtracting the pOH value of 2.72 from 14, resulting in a pH of 11.3. This result is consistent with the expected basic nature of a weak base like ammonia, which raises the pH above the neutral value of 7. The paragraph emphasizes the mathematical nature of the problem while also providing a clear understanding of the chemical concepts involved.

Mindmap
Keywords
💡weak base
A weak base is a substance that partially accepts protons (H+ ions) in a solution, resulting in a limited increase in hydroxide ion (OH-) concentration. In the context of the video, ammonia (NH3) is an example of a weak base because it only partially reacts with water, accepting hydrogen ions and forming hydroxide ions, which leads to a basic solution but not as strongly as a strong base would.
💡equilibrium reaction
An equilibrium reaction is a chemical process in which the forward and reverse reactions occur at the same rate, leading to a balance of reactants and products. In the video, the reaction between ammonia and water is described as an equilibrium reaction because ammonia can both accept a proton from water and release it, maintaining a dynamic balance.
💡Bronsted-Lowry definition
The Bronsted-Lowry definition is a concept in chemistry that describes acids as proton donors and bases as proton acceptors. In the video, this definition is used to classify ammonia as a base because it accepts a proton from water, forming ammonium (NH4+) and hydroxide (OH-) ions.
💡Arrhenius definition
The Arrhenius definition of acids and bases refers to acids as substances that increase the concentration of hydrogen ions (H+) in a solution and bases as those that increase the concentration of hydroxide ions (OH-). In the video, it is mentioned that according to the Arrhenius definition, ammonia increases the hydroxide concentration in the solution, thus qualifying as an Arrhenius base.
💡pKb
pKb is the negative logarithm of the base dissociation constant (Kb) and is used to measure the strength of a base, similar to how pKa is used for acids. A lower pKb value indicates a stronger base. In the video, the pKb of ammonia is given as 4.75, which is used to calculate the equilibrium constant for the reaction involving ammonia.
💡Kb
Kb is the base dissociation constant, which quantifies the strength of a base in water. A higher Kb value indicates a stronger base. In the video, Kb is calculated for ammonia using the given pKb value, and this constant is crucial for determining the hydroxide and ammonium ion concentrations resulting from the reaction of ammonia with water.
💡concentration of hydroxide
The concentration of hydroxide refers to the amount of hydroxide ions (OH-) present in a solution. In the context of the video, the concentration of hydroxide is important for determining the pH of the solution, as it reflects the basicity introduced by the weak base ammonia.
💡pOH
pOH is the negative logarithm of the hydroxide ion concentration and is used to describe the basicity of a solution. A lower pOH value indicates a higher concentration of hydroxide ions and thus a more basic solution. In the video, the pOH is calculated from the concentration of hydroxide ions produced by the reaction of ammonia with water.
💡pH
pH is a measure of the hydrogen ion concentration in a solution and is used to express its acidity or basicity. A pH above 7 indicates a basic solution. In the video, the pH of the solution is determined by calculating the pOH from the hydroxide ion concentration and then using the relationship between pH and pOH to find the final pH value.
💡0.2 molar NH3
The term '0.2 molar NH3' refers to the initial amount of ammonia used in the example, where 0.2 moles of ammonia (NH3) are dissolved in the solution. This initial concentration is crucial for calculating the equilibrium concentrations of ammonia, ammonium ions, and hydroxide ions in the solution.
💡quadratic equation
A quadratic equation is a polynomial equation of degree 2, which typically takes the form ax^2 + bx + c = 0. In the video, a quadratic equation is used to solve for the unknown concentration variable 'x', representing the concentration of hydroxide and ammonium ions resulting from the reaction of ammonia with water.
Highlights

The discussion begins with an example of a weak base, ammonia, and its chemical structure.

Ammonia is identified as a weak base due to its tendency to accept hydrogen from water, resulting in an increase in hydroxide concentration.

The reaction involving ammonia is described as an equilibrium reaction, not proceeding in just one direction.

The Bronsted-Lowry definition is introduced, highlighting ammonia as a proton acceptor, which classifies it as a base.

Ammonia turns into ammonium, a positively charged ion, when it accepts a proton from water.

The equilibrium constant (Kb) for the base reaction is determined by using the pKb value from Wikipedia, which is found to be 4.75.

A mathematical calculation is performed to solve for the equilibrium constant Kb, resulting in a value of 1.8 x 10^-5.

An approach to solving the problem is outlined, which involves calculating the hydroxide concentration and subsequently the hydrogen concentration.

The relationship between pOH and pH is discussed, emphasizing that pOH plus pH equals 14 at 25 degrees Celsius.

A method for calculating the hydroxide concentration ([OH-]) in the presence of ammonia is detailed, using the equilibrium constant and the initial concentration of ammonia.

The process of solving for x, representing the concentration of NH4+ and OH-, is described using a quadratic equation.

The concentration of hydroxide ions is found to be 1.888 x 10^-3 M, which is used to determine the pOH.

The pOH is calculated to be 2.72, indicating a higher concentration of hydroxide ions compared to the neutral concentration of 10^-7 M.

The pH of the solution is determined by subtracting the pOH from 14, resulting in a pH of 11.3 for the 0.2 M ammonia solution.

The result confirms the basic nature of ammonia as a weak base, increasing the pH above neutral but not as much as a strong base would.

The process is characterized as more of a math problem than chemistry, emphasizing the quantitative aspect of the calculation.

The example serves to clarify the concepts of weak bases, equilibrium reactions, and the calculation of pH in the context of a chemical solution.

Transcripts

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Chemistry EducationWeak BasesAmmonia SolutionEquilibrium ReactionspH CalculationBronsted-Lowry TheoryAqueous ChemistryChemical EquilibriumMathematical ModelingChemistry Tutorial