Experiment 3: ANALYSIS OF UNKNOWN ACID SAMPLE USING TITRATION METHOD Date of Experiment: 4 September 2012 Introduction
An acid-base titration is a procedure used in quantitative chemical analysis to determine the concentration of either an acid or a base. Titration is the slow addition of an acid (or a base) of known concentration from a burette (a narrow graduated cylinder) to a base (or an acid) of unknown concentration fin an Erlenmeyer flask. There are two types of acids: strong acid and weak acid, and also two types of bases: strong base and weak base. Therefore four types of acid-base titrations are possible: strong acid-strong base, strong base-weak acid, weak acid-strong base and strong acid-weak base titrations.
Acid + Base Salt + Water
The end point occurs when the stoichiometric amount of base (or acid) has been added to the acid (or the base). The end point is frequently detected using a visual indicator. An acid-base indicator is a substance, which changes colour with pH. For each type of acid-base titration, an appropriate indicator must be chosen. However, the end point can also be determined potentiometrically using a pH meter or by a conductometric method. At this point, all the acid has been neutralized and neither excess base nor excess acid is present in the solution. The solution consists of salt and water only. That is why acid-base titrations are also called neutralization titrations (Sienko and Plane 1957, 340-343). Neutralization reactions in experiments:
NaOH(aq) + KHP(aq) Na+ + KP- +H2O
NaOH(aq) + CH3COOH CH3COO- + Na+ +H2O
- Some indicators (including Phenophthalein)
| pH at which colour changes
| Colour at lower pH
| Colour at higher pH
This is a large molecule (C8H5KO4) with a molar mass of 204.2 g/mol. Instead of writing the whole formula, the formula is abbreviated as KHP, where “P” stands for the phtahalate ion, C8H4O42-, not for phosphorus. KHP is an acidic substance, with the ionizing hydrogen being set forward in the formula for emphasis (Rushin 2012).
The main objective is to discover/determine the unknown acid sample using titration method and to calculate volume to neutralize acid sample.
The aim of the experiment is to determine the molarity and percent by weight of acetic acid in the vinegar solution through titration method, using standard solution— sodium hydroxide solution. The standardization of sodium hydroxide solution uses KHP solution and also phenolphthalein (an acid-base indicator). The calculation selection is using formula of Stoichiometry and others.
The aim of the experiments is also to learn the ways to conduct experiment safe and properly, and the importance of teamwork and leading skill.
In part A
When sodium hydroxide solution is prepared by dissolving sodium hydroxide pellets with distilled water, colourless solution will be observed. In part B
If 3-5 drops of phenolphthalein are added to 20 mL of 0.1 M KHP solution, a pink solution will be found. The prepared sodium hydroxide solution is added from burette into the KHP solution in the flask and it is swirled constantly during the process. Then the colour changes in the solution will be seen. The pink colour solution will turn into a pale pink colour one at the end point. In part C
The 30 mL vinegar solution is covered with a watch glass to prevent evaporation. About 2 mL of vinegar sample is diluted with 100 mL of distilled water. Phenolphthalein indicator is used. Then when it is titrated with sodium hydroxide solution, a pale pink solution will be observed at the end point.
Task A: Preparation of the approximately 0.1 M sodium hydroxide solution 1
| Take out sodium hydroxide pellets and...
References: Acetic acid MSDS. 2012. “Material Safety Data Sheet Acetic acid MSDS.” Accessed September 3,
Chemlab. 1997-2000. “Chemlab-glassware-burets.” Darmouth College.
Sienko, Michell and Robert Plane. 1957. Chemistry. McGraw-Hill Book Company, Inc.
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