Find the Ka of an acid (Given pH) (0.1 M Hypochlorous acid) EXAMPLE

chemistNATE
6 Mar 201204:49
EducationalLearning
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TLDRThe video script explains the process of calculating the equilibrium constant (KA) for a weak acid, using the example of HClO. It emphasizes understanding the dissociation of the acid into H+ and ClO- ions and using the given pH to determine the concentration of H+. The script then illustrates how to set up an ICE (Initial, Change, Equilibrium) table and solve for the unknown concentration (X), which represents the concentration of H+. Finally, it demonstrates how to use the concentrations at equilibrium to calculate KA, resulting in a legitimate value for a weak acid.

Takeaways
  • 📚 KA stands for the equilibrium constant for the dissociation of an acid.
  • 🧪 When given pH and concentration, the goal is to calculate the acid's KA value.
  • 🌟 HClO dissociates in water to form H+ and ClO- ions, with hypochlorite being the anion.
  • 📈 The equilibrium expression for the reaction involves concentrations of products and reactants at equilibrium.
  • 🔍 An ICE (Initial, Change, Equilibrium) table helps to set up the reaction and find the concentration of each species.
  • 💡 The concentration of H+ (X) is found by using the formula 10^(-pH), in this case, 10^(-4.23).
  • 🔧 The concentration of H+ (X) is equal to the concentration of ClO- (X) at equilibrium.
  • 📊 The concentration of HClO at equilibrium is the initial concentration minus the change (X).
  • 🧠 The KA value is calculated by the product of the concentrations of the ions (H+ and ClO-) divided by the concentration of the undissociated acid (HClO).
  • 🔢 The calculated KA value of 3.47 × 10^(-8) is a legitimate value for a weak acid.
  • 🎓 This method is an effective way to calculate the KA of a weak acid when given the pH and concentration.
Q & A

  • What does KA stand for in the context of chemistry?

    -KA stands for the equilibrium constant for the dissociation of an acid. It is a measure of the strength of an acid, indicating how readily the acid donates protons (H+ ions) in solution.

  • What is the relationship between pH and H+ ion concentration?

    -The pH of a solution is the negative logarithm (base 10) of the concentration of H+ ions. The formula is pH = -log[H+]. A lower pH value indicates a higher concentration of H+ ions, meaning the solution is more acidic.

  • How is the dissociation of HCl represented in an aqueous solution?

    -HCl is a strong electrolyte and dissociates completely in water to form H+ and Cl- ions. The equation for this dissociation is: HCl → H+ + Cl-. Since it is a strong acid, the reaction is not reversible and does not use a reversible arrow.

  • What is the hypochlorite ion?

    -The hypochlorite ion is ClO-. However, in the context of the script, it seems there was a confusion between chloride (Cl-) and hypochlorite (ClO-). The chloride ion is the conjugate base formed when a strong acid like HCl dissociates in solution.

  • How do you calculate the concentration of H+ ions given the pH value?

    -Given the pH value, you can calculate the concentration of H+ ions using the formula: [H+] = 10^(-pH). For example, if the pH is 4.23, the concentration of H+ ions would be 10^(-4.23) M.

  • What is an ICE table and how is it used in calculating KA?

    -An ICE table, which stands for Initial, Change, and Equilibrium, is a method used to keep track of the concentrations of reactants and products at different stages of a chemical equilibrium. It helps in setting up the equilibrium expression and solving for the unknown variables, such as the equilibrium constant (KA).

  • Why is it important to know the value of KA for an acid?

    -Knowing the value of KA for an acid is important because it provides a quantitative measure of the acid's strength. A larger KA value indicates a stronger acid, which means it dissociates more completely in solution. This information is crucial in various chemical calculations and understanding reaction dynamics.

  • How does the concentration of HCl initially and at equilibrium relate to the calculation of KA?

    -In the calculation of KA, the initial concentration of HCl is usually given, and the change in concentration represents the amount of HCl that has dissociated. At equilibrium, the concentration of HCl will be the initial concentration minus the change (X). These values are used in the equilibrium expression to solve for KA.

  • What is the significance of the equilibrium constant (KA) in predicting the behavior of an acid in solution?

    -The equilibrium constant (KA) is significant as it predicts the extent to which an acid will dissociate in solution. A large KA value indicates that the acid dissociates largely, forming more products at equilibrium, which means the acid is strong. Conversely, a small KA value suggests limited dissociation, indicating a weak acid.

  • How does the concentration of H+ ions affect the pH value of a solution?

    -The concentration of H+ ions directly affects the pH value of a solution. As the concentration of H+ increases, the pH value decreases, making the solution more acidic. Conversely, a lower concentration of H+ ions results in a higher pH value, indicating a less acidic (more basic) solution.

  • What is the role of the calculator in determining the concentration of H+ ions from the pH value?

    -A calculator is used to compute the antilog (the number obtained by taking the logarithm to the base 10) of the negative pH value to find the concentration of H+ ions. For example, for a pH of 4.23, you would calculate 10^(-4.23) to find the H+ ion concentration.

  • What is the process of calculating the KA of an acid from a given pH and concentration?

    -To calculate the KA of an acid from a given pH and concentration, you first determine the concentration of H+ ions using the pH value (using the formula [H+] = 10^(-pH)). Then, using an ICE table, you set up the equilibrium expression for the dissociation of the acid. By solving for the unknown concentration variables (X), you can calculate the ratio of the product concentrations to the reactant concentration at equilibrium, which gives you the value of KA.

Outlines
00:00
📚 Calculating the Acid Dissociation Constant (Ka)

This paragraph delves into the process of calculating the Ka of an acid when given its pH at a specific concentration. It begins with a brief discussion on the meaning of Ka, which is the equilibrium constant for the dissociation of an acid. The example of HClO dissociating into H+ and ClO- ions is used to illustrate the concept. The paragraph then explains how to use an ice table to determine the concentration of each species at equilibrium, highlighting the importance of understanding the dissociation process. It proceeds to calculate the concentration of H+ using the given pH value, and subsequently uses this information to find the Ka value. The explanation is thorough, walking through each step of the calculation and emphasizing the relevance of understanding these chemical concepts.

Mindmap
Keywords
💡KA
KA stands for the equilibrium constant for the dissociation of an acid in solution. It is a measure of the strength of an acid, indicating how readily the acid donates protons (H+ ions) in water. In the context of the video, the speaker is discussing how to calculate the KA of a weak acid given its pH at a particular concentration, emphasizing the importance of understanding acid dissociation in chemistry.
💡pH
pH is a numerical measure of the acidity or basicity of a solution, defined as the negative logarithm (base 10) of the activity of hydrogen ions (H+) in the solution. A lower pH value indicates a higher concentration of H+ ions, meaning the solution is more acidic. In the video, the speaker uses the given pH to find the concentration of H+ ions, which is crucial for calculating the KA of the acid.
💡Hypochlorite ion
The hypochlorite ion (ClO-) is a chlorine-containing anion resulting from the dissociation of certain chlorine compounds in water, such as hypochlorous acid (HClO). It plays a significant role in the context of the video as it is one of the products formed when a weak acid like HClO dissociates in solution.
💡Equilibrium expression
An equilibrium expression is a mathematical equation that relates the concentrations of reactants and products in a chemical reaction at equilibrium. It is constructed using the concentrations of each species involved, raised to the power of their stoichiometric coefficients in the balanced chemical equation. In the video, the equilibrium expression for the dissociation of HClO is used to calculate the KA constant.
💡Concentration
Concentration in chemistry refers to the amount of a particular substance present in a given volume of solution. It is typically measured in moles per liter (M) and is crucial for understanding chemical reactions, including acid dissociation. The video script discusses the concentrations of HClO, H+, and ClO- at equilibrium to calculate the KA value.
💡ICE table
An ICE table (Initial, Change, Equilibrium table) is a method used in chemistry to set up and solve problems involving chemical equilibria. It breaks down the reactants, products, and the changes in concentration during the reaction, making it easier to calculate equilibrium constants like KA. The video script mentions using an ICE table to figure out the concentrations at equilibrium.
💡Dissociation
Dissociation is the process by which a compound splits into two or more smaller molecules or ions when dissolved in a solvent. In the context of the video, it specifically refers to the splitting of weak acids like HClO into their constituent ions (H+ and ClO-) in an aqueous solution.
💡Weak acid
A weak acid is an acid that does not completely dissociate in water, meaning only a small fraction of its molecules release protons (H+ ions) into the solution. Weak acids have a lower tendency to donate protons compared to strong acids, resulting in lower concentrations of H+ ions and higher pH values. The video focuses on calculating the KA for a weak acid, illustrating the process with HClO as an example.
💡Equilibrium
Chemical equilibrium is the state in which the concentrations of reactants and products remain constant over time because the forward and reverse reaction rates are equal. At equilibrium, there is no net change in the concentrations of the reactants and products, although the reactions continue to occur. The video script discusses the equilibrium constant KA, which describes the equilibrium state of an acid's dissociation in water.
💡10 to the power of negative
The expression '10 to the power of negative' is used in scientific notation to represent very small numbers, typically those less than 1. It indicates the reciprocal of a whole number power of 10, meaning that the decimal point is moved a certain number of places to the left. In the context of the video, this notation is used to express the concentration of H+ ions given the pH value.
💡Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances into another, involving the breaking and forming of chemical bonds. In the video, the chemical reaction of interest is the dissociation of a weak acid like HClO into its ions. Understanding this reaction is essential for calculating the equilibrium constant KA.
Highlights

Calculating the KA of an acid using given pH and concentration

Understanding the meaning of KA as the equilibrium constant for the dissociation of an acid

Dissociation of HClO in solution into H+ and ClO- ions

Establishing the equilibrium expression for the reaction

Using an ICE (Initial, Change, Equilibrium) table to determine concentrations at equilibrium

Setting up the equilibrium concentrations with HClO initially at 0.1 and the other concentrations as zero

Finding the concentration of H+ using the given pH value

Calculating the concentration of H+ as 10 to the power of negative pH

Deducing that the concentration of H+ (X) is 5.89 times 10 to the negative 5

Expressing the relationship between the concentrations of H+, ClO-, and HClO at equilibrium

Recognizing that the concentration of HClO is negligible compared to 0.1

Calculating the KA value using the known concentrations and the equilibrium expression

Arriving at a KA value of 3.47 times 10 to the negative 8

The method provides a legitimate value for the KA of a weak acid

The process demonstrates a comprehensive approach to solving chemical equilibrium problems

Transcripts

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