Friday, January 9, 2009

AS level chemistry practical (GCE)

Chemistry 101 Laboratory
Experiment 10: Acid/ Base Titrations
Pre-lab Questions
1. How would your calculated molarity of NaOH be affected by each of the following errors? Elaborate.

a.) The buret is not rinsed with solution before filling.
b.) Some solution splashes out of the flask during titration.
c.) You go past the end point in the titration.
2. How would your calculated value for the equivalent weight of the acid be affected by each of the following errors? Elaborate.
a.) Some acid is spilled out of the flask after it is weighed.
b.) Some solution splashes out of the flask during titration.
c.) You go past the end point of the titration.
3. What is the effect of allowing a solution of NaOH to remain in contact with the air in the laboratory? Be specific as to the two, main effects and their sources.
The reaction of an acid and a base in water involves the reaction of hydroxide and hydrogen ions to form water (neutralization) and the formation of the resulting salt from the conjugate base and acid. The latter may or may not be soluable.
In general, the reaction is given by the following reaction for simple acids and bases:
HA + HOX <-> H2O + A- + X+
In this example, just one hydrogen and one hydroxl ion were available for the reaction. The ionization of the acid therefore produces one "equivalent" (one mole per mole) of hydrogen ions and ionization of the base produces one equivalent of hydroxyl ions. ( It is important to remember that, in water, the hydrogen ion and hydroxyl ion are the strongest acid and base, respectively. For this reason, ionization of any acid or base in water leads to the production of soley these acidic and basic ions.)
It is possible that one mole of the acid or base will produce more than one mole (one equivalent mole) of hydrogen ions or hydroxyl ions. An example of this is the diprotic acid sulfuric H2SO4.
In this instance, a total of 2 moles of hydrogen ions are produced when the acid is completely ionized to the sulfate.
When a titration is carried out, an acid or base of unknown concentration is allowed to neutralize a base or acid known to produce a known number of moles of hydroxyl or hydrogen ions.
As the acid or base is added to the solution of a known number of moles, the acids and bases react to neutralize one another.
At the point where the number of moles of acid equals the number of moles of base, we have reached what is called equivalence.
This is also known as the equivalence point and is shown either by a indicating dye or pH meter.
At the equivalence point we know the number of moles of acid or base reacted (from the fact that we know the number of moles of base or acid originally present from the sample of known amount) and the number of liters of the unknown which were added to neutralize the solution.
From the definition of molarity we have the following relationship:
Equivalent molarity = (# moles originally )/ ( liters of unknown added)
Notice that this differs slightly from our usual definition of molarity.
The reason is that we have not taken into account the fact that our unknown acid or base may have more than one equivalent per mole.
In today's experiment, you will deal with a monoprotic acid and a simple hydroxyide (NaOH) so that the concentration you obtain will be the true molarity of the base.
One of the important concepts in any aspect of chemistry in which quantitative answers are required is that of the primary standard.
This is an agreed upon set of conditions, chemical compound, or anything else which is agreed to represent a measure to which other things are compared.
One such standard concerns hydrogen ion concentration and is the use of potassium hydrogen phthalate (KHC8H4O4, molecular weight 204.224) as a standard for the concentration of hydrogen ions in a solution.
One mole of this salt produces one mole of hydrogen ions in one liter of water.
A second thing which is required in the use of a primary standard is a means of knowing when all of the (in this case) hydrogen ions from the standard have been neutralized with the base.
One way of doing this is to use an indicator dye.
These are organic chemicals which are also weak acids and bases. On their ionization or protonation, they undergo a chemical change which also results in a change in color.
One such dye is phyenylphtalene, a close relative of the primary standard you are using. This dye is clear in acidic solutions, slightly pink in neutral pH solutions, and dark purple in basic solutions.
Addition of a small amount of the dye to the solution you are testing allows you to determine when an equivalent number of acids and bases were present.

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