12: Molar Mass Determination Using Freezing Point Depression
- Page ID
- 514174
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Demonstrate the colligative property of freezing-point depression.
- Measure the change in freezing point when a known solute is dissolved in a solvent.
- Determine the molar mass of the solute experimentally.
INTRODUCTION
When a solute is dissolved in a solvent, the freezing point of the resulting solution is lower than that of the pure solvent. This phenomenon, known as freezing point depression, is a colligative property. Colligative properties depend on the concentration of solute particles in a solution rather than the chemical identity of the solute itself.
The following equation expresses the relationship between freezing-point depression and the concentration of a solution:
\(\Delta T = K_f \times m\)
Where:
- \(\Delta T\) is the freezing-point depression, defined as the difference between the freezing point of the pure solvent and that of the solution.
- \(K_f\) is the freezing-point-depression constant, specific to each solvent. In this experiment, lauric acid is the solvent, with \(K_f = 3.9\ \text{°C·kg/mol}\).
- \(m\) is the molality of the solution, defined as the number of moles of solute per kilogram of solvent.
This experiment is divided into two parts:
- Determining the Freezing Point of Pure Lauric Acid, \(\ce{CH3(CH2)10COOH}\): The freezing point of the pure solvent will be determined experimentally by observing its cooling curve.
- Determining the Freezing-Point Depression of a Solution to Calculate the Molar Mass of the Solute: A known mass of the solute, benzoic acid \(\ce{C6H5COOH}\), will be added to a known mass of lauric acid. By comparing the freezing points of the pure solvent and the solution, the freezing-point depression \(\Delta T\) can be calculated. This value, along with \(K_f\) and the mass of the solvent, will be used to determine the molar mass of the solute.
- 12.1: Molar Mass Determination Using Freezing Point Depression - Experiment
- This page details laboratory safety precautions, including wearing safety goggles and careful handling of hot equipment and glassware. It lists necessary equipment and chemicals like lauric and benzoic acids. The experimental procedure is divided into three parts: setting up data collection to determine the freezing point of lauric acid, measuring the freezing point depression of a benzoic acid/lauric acid solution, and providing chemical disposal guidelines.
- 12.2: Molar Mass Determination Using Freezing Point Depression - Pre-lab
- This page explains colligative properties of solutions, emphasizing that they rely on solute particle quantity rather than identity. Key concepts include vapor pressure lowering, boiling point elevation, and freezing point depression, which correlates with solute concentration. Each solvent has a unique freezing point depression constant, \(K_f\). Cooling curves demonstrate temperature variations during freezing, where pure solvents behave differently than solutions.
- 12.3: Molar Mass Determination Using Freezing Point Depression - Data and Report
- This page details an experiment to identify the freezing point of pure lauric acid and analyze the freezing point depression in a benzoic acid/lauric acid solution. It covers data collection on mass and freezing points, along with calculations for various related metrics. Additionally, it includes post-lab questions exploring potential errors, the importance of the cooling curve plateau, solvent effects, intermolecular forces, and real-world implications.