The City University of New York (CUNY)
LaGuardia Community College

Course: Fundamentals of Chemistry II / Section: SCC202.1761
Professor Midas

Student Name: Khushbu Patel
Laboratory #7 – Determination of Ka for Weak Acids
Laboratory Date: April 25, 2012

Objective
Titrate a solution of unknown acid to the half-reacted point and use the pH of the solution at that point to determine ka for the unknown acid.

Titrate a solution containing a weighed amount of solid unknown acid to the half-reacted point and use the PH of the solution at that point to determine Ka for the unknown Ka for the unknown acid.

Introduction
When an uncharged weak acid is added to water, a heterogeneous equilibrium forms in which aqueous acid molecules, HA(aq), react with liquid water to form aqueous hydronium ions and aqueous anions, A-(aq). The latter are produced when the acid molecules lose H+ ions to water.
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Water is not included in the acid-dissociation equilibrium expression because the [H2O] has no effect on the equilibrium.

Acid + Base→ Salt+ Water

Materials Used
10, 50 and 100-mL graduated cylinder
125-mL flask
3.5-mL of 6M NaOH Solution
Distilled Water
Glass stirring rod
Beaker
50-mL of Unknown Acid Solution
25- or 50-mL buret
Ringstand with buret clamp
10-mL pipet
125mL flask
Phenolphthalein indicator
Short range pH paper pH meter Procedure A. Determination of Ka for an Unknown Acid in Solution * Measure and deposit approximately 50-mL of unknown acid into a clean dry 100-mL beaker * Wash and rinse a 25- or 50-mL buret with 5-mL of the previously prepared NaOH solution * Fill buret with NaOH (previously prepared) solution and mount on ringstand * Wash and rinse a 10-mL pipet with the unknown acid solution * Into a clean 125-mL flask, pipet 10.00-mL of