3: Acid Dissociation Constant
- Page ID
- 516588
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)PURPOSE
- To titrate an unknown weak acid using a pH probe.
- To use the titration curve to determine the pKa (and therefore the Ka) of the acid and determine a likely identity of the acid.
INTRODUCTION
This lab experiment focuses on titrating an unknown weak acid using a pH probe. The primary goal is to determine the acid dissociation constant (Ka) of the unknown weak acid.
The procedure involves titration of the acid using a 0.1 M NaOH solution as the titrant, a pH sensor, and a magnetic stirrer to ensure mixing.
Data collection begins by slowly adding the NaOH solution to a known volume of the unknown weak acid, while the pH is continuously monitored and recorded. The titration continues until the pH is over 11.
The Ka value is calculated using the Henderson-Hasselbalch equation. Specifically, at the half-equivalence point of the titration, the pH is equal to the pKa of the weak acid. From the pKa, the Ka can then be determined. The experiment also involves analyzing the titration graphs to estimate the concentration of the unknown acid from the equivalence point.
Calculating the Ka
For a monoprotic weak acid, HA, the Henderson-Hasselbalch equation is
\[ \text{pH}=\text{p}K_{\text{a}}+\log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right)\]
where \( [\text{HA}] \) and \( [\text{A}^-] \) are the equilibrium molar concentrations of the weak acid and its conjugate base, respectively.
At the half-equivalence point, \( V_{1/2} = \frac{1}{2} V_{\text{eq}} \),
\[ [\text{HA}] = [\text{A}],\ \ \text{so}\ \ \frac{[\text{A}]}{[\text{HA}]} = 1,\ \ \text{and}\ \ \log\left( \frac{[\text{A}]}{[\text{HA}]} \right) = 0 \]
Therefore, at the half-equivalence point,
\[ \text{pH}=\text{p}K_{\text{a}} \]
The \( K_{\text{a}} \) can be calculated using the definition of \( \text{p}K_{\text{a}} \),
\[ \text{p}K_{\text{a}} = -\log{K_{\text{a}}},\ \ \text{so}\ \ K_{\text{a}} = 10^{-\text{p}K_{\text{a}}} \]
Henderson-Hasselbalch Equation:
\[ \text{pH}=\text{p}K_{\text{a}}+\log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right)\]
Definition of pK_a:
\[ \text{p}K_\text{a} = -\log{K_\text{a}}\ \ \ \ \ \text{or}\ \ \ \ \ K_\text{a} = 10^{-\text{p}K_\text{a}} \]
- 3.1: Acid Dissociation Constant - Experiment
- This page outlines safety precautions for handling caustic acids and sodium hydroxide, alongside waste disposal instructions. It specifies required equipment and chemicals, including a pH probe and monoprotic weak acid. The experimental procedure details the titration steps with 0.1 M NaOH, highlighting careful mixing and accurate pH recording until pH 11 is achieved. Additionally, it emphasizes rinsing the pH sensor and proper waste disposal practices.
- 3.2: Acid Dissociation Constant - Pre-lab
- This page covers acid-base chemistry, specifically the dissociation of a monoprotic acid (HA) and its equilibrium constant (Ka). It includes tasks such as writing the dissociation reaction, deriving the Ka expression, and finding concentrations at the half-equivalence point in a titration.
- 3.3: Acid Dissociation Constant - Data and Report
- This page covers a lab activity on titration experiments to analyze an unknown acid. Students perform two titrations, document data in tables, construct titration curves, and calculate equivalence points and pKa values. They also evaluate the repeatability of results, determine the acid's concentration, and hypothesize its identity based on calculated pKa. The focus is on data analysis and interpretation in acid-base chemistry.