How to find the Ka of an acid when given pH

Melissa Maribel
28 Feb 202204:00
EducationalLearning
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TLDRThe video script outlines a step-by-step process for determining the acid dissociation constant (Ka) of chloroacetic acid using a given molarity and pH. It emphasizes the importance of understanding equilibrium concentrations and the ICE table method for calculating changes in concentration. The script also highlights the significance of significant figures in pH readings and their relation to hydronium ion concentration. Finally, it instructs on how to use these values in the Ka expression to find the Ka of the acid, offering a comprehensive guide for acid-base equilibria.

Takeaways
  • 🔍 The goal is to calculate the Ka value for chloroacetic acid given its molarity and pH.
  • 📈 Start by using the pH value (1.86) to find the concentration of hydronium ions, considering significant figures.
  • 🌟 The dissociation equation of chloroacetic acid in water helps identify the acid and its conjugate base.
  • 📊 Build an ICE (Initial, Change, Equilibrium) table to organize the concentrations of reactants and products at equilibrium.
  • ⚖️ The initial concentration of the acid is provided, but the initial concentration of hydronium ions is assumed to be zero.
  • 🔄 The change in concentration (x) is determined to be -x for the reactant and +x for the products, leading to an equilibrium concentration of 0.014 M.
  • 📝 Write the Ka expression without including liquids or solids, only using the concentrations of the products and reactants in the solution.
  • 🧮 Plug the equilibrium concentrations into the Ka expression to solve for Ka, which can be expressed in standard form or scientific notation.
  • 📚 The process involves understanding the relationship between pH and Ka, and how to apply it to find the acid dissociation constant.
  • 🌐 The script provides a step-by-step guide for solving acid-base equilibrium problems, emphasizing the importance of following a structured approach.
  • 💡 Additional resources are suggested for further understanding of acid-base equilibria, indicating that there is more to learn beyond the given example.
Q & A

  • What is the molarity of the chloroacetic acid solution given in the example?

    -The molarity of the chloroacetic acid solution is 0.15 M.

  • What is the pH value of the solution mentioned in the transcript?

    -The pH value of the solution is 1.86.

  • What does the 'ka' value represent in the context of the script?

    -The 'ka' value represents the acid dissociation constant for chloroacetic acid, which is a measure of the strength of the acid in donating a proton (H+).

  • How many significant figures should the concentration of hydronium ions have based on the pH value?

    -The concentration of hydronium ions should have the same number of significant figures as the pH value, which is two in this case.

  • What is the relationship between an acid and a base according to the script?

    -An acid donates a proton (H+) to a base, which accepts the proton to form the conjugate acid.

  • What is the initial concentration of hydronium ions in the ICE table?

    -The initial concentration of hydronium ions is not given, so it is labeled as zero in the ICE table.

  • What does 'x' represent in the ICE table?

    -'x' represents the change in concentration of the reactants and products at equilibrium.

  • How is the concentration at equilibrium for the acid and its products calculated?

    -The concentration at equilibrium for the acid is found by subtracting 'x' from the initial concentration. The concentration for the products is 'x'. In this case, both are 0.014 M.

  • What should be considered when writing the Ka expression?

    -When writing the Ka expression, liquids or solids should not be included. Only the products (conjugate base and H+) are placed on the top, and the reactant (un dissociated acid) is on the bottom.

  • How is the Ka value calculated using the equilibrium concentrations?

    -The Ka value is calculated by multiplying the concentrations of the products (conjugate base and H+) and dividing by the concentration of the reactant, with all values taken from the equilibrium concentrations.

  • What is the significance of the ICE table in the context of the script?

    -The ICE table (Initial, Change, Equilibrium) is a method used to visualize and calculate the changes in concentration of reactants and products at equilibrium for a chemical reaction, such as the dissociation of an acid.

Outlines
00:00
📚 Determining Ka for Chloroacetic Acid

This paragraph introduces the problem of calculating the Ka value for a 0.15 molar solution of chloroacetic acid with a pH of 1.86. The main objective is to find the equilibrium concentrations and then use them in the Ka expression. The paragraph outlines a step-by-step approach, starting with identifying the given values (molarity and pH) and the target (Ka). It emphasizes the importance of understanding the dissociation equation, forming the conjugate base, and setting up an ICE (Initial, Change, Equilibrium) table to determine the concentrations at equilibrium. The paragraph concludes with the calculation of Ka using the equilibrium concentrations, highlighting the exclusion of liquids and solids in the equilibrium expression.

Mindmap
Keywords
💡Molar Solution
A molar solution refers to a way of expressing the concentration of a solute in a solution. It is defined as the number of moles of solute per liter of solution. In the video script, the example starts with a 0.15 molar solution of chloroacetic acid, setting the stage for the calculation that follows. This initial concentration is crucial as it is used throughout the process to determine the dissociation constant (Ka) for the acid, showcasing how solution concentration directly influences acidity and reaction equilibrium.
💡pH
pH is a scale used to specify the acidity or basicity of an aqueous solution. It is calculated as the negative logarithm of the hydrogen ion concentration. In the script, a pH of 1.86 indicates a highly acidic solution, which is crucial for calculating the concentration of hydronium ions in the solution. Understanding the pH helps in determining the extent of the acid's dissociation and is fundamental in the steps to calculate the Ka value.
💡Ka
Ka, or the acid dissociation constant, is a quantitative measure of the strength of an acid in solution. It is derived from the equilibrium concentrations of the components in the acid dissociation reaction. The video's main objective is to calculate Ka for chloroacetic acid, illustrating the process of determining the strength of an acid based on its dissociation in water. The calculation of Ka is pivotal in understanding acid strength and is a key focus of the script.
💡Hydronium Ions
Hydronium ions (H3O+) are the positive ions formed when acids dissolve in water, and their concentration is directly related to the acidity of the solution. In the script, finding the concentration of hydronium ions using the given pH is an essential step that directly impacts the calculation of Ka. This demonstrates the relationship between pH, hydronium ion concentration, and the acid's dissociation process.
💡Dissociation Equation
The dissociation equation represents the chemical reaction in which an acid breaks down into its ions in a solution. In the context of the script, the dissociation equation for chloroacetic acid involves its separation into H+ and its conjugate base in water. This step is foundational in understanding the reaction mechanism and sets the stage for the subsequent equilibrium calculations, illustrating the transformation of reactants to products.
💡ICE Table
An ICE table (Initial, Change, Equilibrium) is a systematic method used to calculate the concentrations of reactants and products at equilibrium in a chemical reaction. The script details constructing an ICE table for the acid dissociation reaction, highlighting how it helps organize and solve for unknown values. By tracking the changes from initial concentrations through to equilibrium, the ICE table facilitates the calculation of Ka.
💡Equilibrium Concentrations
Equilibrium concentrations refer to the amounts of reactants and products present when a chemical reaction reaches a state where no further changes occur in their concentrations. In the script, determining these concentrations is crucial for calculating Ka. The narrative demonstrates how equilibrium concentrations are derived and used, emphasizing their importance in understanding chemical reactions and acid-base equilibria.
💡Significant Figures
Significant figures refer to the digits in a number that carry meaning contributing to its measurement accuracy. The script mentions the significance of adhering to the rule of significant figures in relation to the pH measurement, illustrating the precision required in chemical calculations. This ensures the accuracy of calculated concentrations and the final Ka value, highlighting the attention to detail needed in scientific analysis.
💡Conjugate Base
A conjugate base is what remains of an acid molecule after it has donated a proton in a chemical reaction. In the video script, the formation of the conjugate base from chloroacetic acid is a critical part of understanding the acid's dissociation and subsequent equilibrium. This concept helps illustrate the acid-base pair relationship and is integral to the calculations and concepts discussed.
💡Equilibrium Expression
The equilibrium expression for a chemical reaction is a formula that relates the concentrations of the reactants and products at equilibrium. In the script, the equilibrium expression is derived for the dissociation of chloroacetic acid, which is used to calculate Ka. This step is essential in quantifying the reaction's dynamics and the acid's strength, showcasing the direct application of equilibrium principles in chemistry.
Highlights

The example involves a 0.15 M molar solution of chloroacetic acid with a pH of 1.86.

The goal is to find the value of the acid dissociation constant (Ka) for chloroacetic acid.

The first step is to identify the given values and the objective, which is to find Ka using equilibrium concentrations.

The second step involves calculating the concentration of hydronium ions (H+) using the given pH value.

Significant figures (sig figs) rules for pH dictate the number of decimal places in the concentration calculation.

The dissociation equation of chloroacetic acid in water is written, identifying the acid and its conjugate base.

An ICE (Initial, Change, Equilibrium) table is constructed to organize the concentrations of reactants and products.

The initial concentration of the acid is given, and the initial concentration of hydronium ions is assumed to be zero.

The change in concentration (x) is determined to be 0.014 M, representing the dissociation of the acid.

Equilibrium concentrations are calculated by subtracting the change in concentration from the initial concentration.

The Ka expression is written without including liquids or solids in the equilibrium expression.

Equilibrium concentrations are plugged into the Ka expression to solve for Ka.

Ka can be expressed as a number or in scientific notation, both are considered correct.

The process described is a step-by-step guide for finding the Ka of an acid using its molarity and pH.

The method can be applied to various acids to determine their dissociation constants.

This approach is fundamental for understanding acid-base equilibria in chemistry.

The transcript provides a comprehensive guide for students learning about acid dissociation constants.

The use of ICE tables is emphasized as a key tool in visualizing and calculating equilibrium concentrations.

The importance of following significant figures rules is highlighted for accurate concentration calculations.

The transcript concludes by directing readers to additional resources for further study on acid-base equilibria.

Transcripts

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Related Tags
Chemistry TutorialAcid-Base EquilibriumKa CalculationChloroacetic AcidPH to KaDissociation EquationICE TableEquilibrium ConcentrationsScientific NotationChemical Analysis