Determining the Molar Concentration of Vinegar by Titration - Le Moyne [PDF]

Determining the Molar Concentration of Vinegar by Titration Objective: Determine the concentration of acetic acid in a vinegar sample Expressing solution concentration. Using volumetric glassware: pipet and buret. Performing a titrimetric analysis.

Background In a titration, the analyte (the substance whose concentration is unknown and sought in the analysis) is reacted with a standard (a substance that reacts with the analyte but whose concentration is known). The analysis uses just enough of the standard to react with all of the analyte, thereby allowing the amount of analyte present to be determined. In this experiment, acetic acid (CH 3 COOH) is the analyte and sodium hydroxide (NaOH) is the standard. The reaction is: CH 3 COOH(aq) + NaOH(aq) --> CH 3 COONa(aq) + H 2 O(l) Titration: an analytical procedure involving a chemical reaction in which the quantity of at least one reactant is determined volumetrically. Standard solution: a solution in which the concentration of a solute is precisely known. Usually it is the volume of the standard solution required to react with a given quantity of an analyte that is precisely determined during a titration. Titration endpoint: The quantity of reactant in the standard solution added during the titration is stoichiometrically equivalent to the quantity of reactant in the analyte at the titration endpoint.

Expressing Solution Concentration mols (solute) molarity = ------------------ = M (mol/L) Liter (solution) 0.493 M NaOH means 0.493 mol NaOH/L mols mols = ------ x L L mols = M x V In a titration procedure, 40.57 mL of 0.493 M NaOH solution was used. How many mols NaOH did this volume of NaOH solution contain? mols = M x V 0.493 mols NaOH mols = ----------------------- x 0.04057 L L mols = 0.0200 mols NaOH

Volumetric glassware: buret and pipet Reading the buret

Using the pipet

Buret reading = 0.76 mL Determining the Volume of Titrant Delivered in a Titration Final buret reading: 49.37 mL Initial buret reading: 0.74 mL volume delivered:

48.63 mL

Procedure You may work in groups: Each person must perform a titration. For those working in groups, each group member will contribute the results of one determination to the group effort. Your reported result will be the average of at least two titrations.

Sample Calculation Calculating the concentration (M) of CH 3 COOH in commercial vinegar. 1. From the balanced chemical equation: mols CH 3 COOH(vinegar) = mols NaOH(titrant) 2. mols NaOH = MNaOH x V NaOH,L (from titration) 0.493 mols NaOH mols Na OH = ------------------------ x 0.04863 L L mols NaOH = 0.240 = mols CH 3 COOH(vinegar) 3. mols CH 3 COOH(vinegar) M CH 3 COOH(vinegar) = ----------------------------volume(vinegar) 0.0240 mols M CH 3 COOH(vinegar) = ---------------- = 0.96 M 0.0250 L Back to the Chemical Principles Lab Schedule.