The use of volumetric flask, burette and pipette in determining the concentration of NaOH solution

     water , the cap was closed up and the flask was rotated several

     times to get a homogenous solution.

4. The solution was poured into a clean and dry beaker , the beaker

     was labeled and was covered with a watch glass and put it aside.

     The volumetric flask was cleaned.

5. The burette was cleaned with the distilled water and was rinsed

     with 5cm3 NaOH solution a few times. The burette was filled

     with NaOH solution by using a funnel. The pipette was cleaned

     and was rinsed a few times with the acid to be used in the

     titration.

6. 20.0cm3 of acid solution was pipette into three clean

    Erlenmeyer flasks.

7. Two drops of phenolphthalein was added into the acid solution.

8. The initial burette reading was recorded and the acid solution

     was titrated with NaOH solution in the burette until the end

     point was reached. The end reading was recorded to obtain the

     volume of NaOH solution was used.

9. The titration was repeated for several times until the NaOH

     solution volumes remain constant within 3/1000 for three

     titrations

Results

Average volume of NaOH used:  26.86 + 26.68 + 26.86 + 26.74        =        26.79mL

                                                       4

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Discussion

An acid-base titration is when you add a base to an acid until the equivalence point is reached which is where the moles of acid equal the moles of base. For the titration of a strong base and a strong acid, this equivalence point is reached when the pH of the solution is seven. The ionic equation is shown below:

• HIn  H+ + In-

An indicator is added to the solution being titrated. The indicator is a substance that changes color when the reaction is complete. Phenolphthalein, which is a commonly used acid‑base indicator, is added to the nitric acid solution in the Erlenmeyer flask. Phenolphthalein has two chemical forms. In acidic conditions, it is in the acid form, which is colorless. In basic conditions, an H+ ion is removed from each phenolphthalein molecule, converting it to its base form, which is red. Furthermore, weak acids are titrated in the presence of indicators which change under alkaline conditions while weak bases are titrated by the indicator that under acidic conditions. Moreover, the indicator does not change color from pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color change occurs over a range of hydrogen ion concentrations. Some common acid base indicators can be used. Tried and true indicators are bromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, neutral red, phenolphthalein and so forth.

The titrant is slowly added to the solution being titrated until the indicator changes color and this stage is called the endpoint. After that NaOH solution is slowly added from the burette until the mixture in the Erlenmeyer flask changes from colorless to red. The OH− ions in the NaOH solution react with the H3O+ ions in the HNO3 solution and it shows as below:

• H3O+(aq)  + OH−(aq)    H2O(l)

As long as excess H3O+ ions in the solution, staying acidic, the phenolphthalein stays mostly in the acid form, and the solution is colorless. When enough NaOH solution is added to react with all of the H3O+ ions, the reaction is complete. When a small amount of extra NaOH solution is added, there will be an excess of hydroxide ions, OH−, in solution.

In this experiment, an acid is a substance that dissociates in water to produce hydrogen ions; a base is a substance that dissociates in water to produce hydroxide ions. Thus, the acid, HCl, and the base, NaOH, dissociate according to the following equations:

• HCl H+ (aq) + Cl- (aq)
• NaOH  Na+ (aq) + OH- (aq)

The hydrated proton, H+ (aq), has been named the hydronium ion and is often written H3O+. The dissociation of HCl, using this convention, is shown as below:

• HCl + H20  H3O+ + Cl-

Water is partially dissociated into ions, is shown as below:

• 2H2O  H3O+ +

There are some precautions have to carry out in this experiment. Firstly, do not allow air bubbles to be trapped in the nozzle of the burette. Usually an air bubble is present in the nozzle of the burette, it must be removed before taking the initial reading. The proper way to read a burette is to read the volume of the liquid at the bottom of the meniscus. The meniscus is the curved upper surface of the liquid in a burette. When viewing the burette with the volumetric scale toward you, follow the lowest point at the surface of the liquid to the wall of the burette. Read the volumetric scale at this point. It is essential that the meniscus be at eye level when determining the volume. Furthermore, always add acid to water, not water to acid, to avoid excess heat formation and spitting of acid. Stir solution while adding acid.

Acid strength use the terms "strong" and "weak" to give an indication of the strength of an acid. The terms strong and weak describe the ability of acid solutions to conduct electricity. If the acid conducts electricity strongly, it is a strong acid. If the acid conducts electricity weakly, it is a weak acid or base. Moreover, acid concentration is well defined that how many acid molecules per unit volume in the solution.

A weak base is a   that does not  fully in an , defining as a chemical base in which  is incomplete. Nonetheless, Insoluble Bases are usually the Metal Oxides such as Magnesium Oxide, Copper (II) Oxide, Lead (II) Oxide, Iron (III) Oxide and Iron (II) Oxide.

Conclusion:

The initial concentration of NaOH is 0.01M. The average volume of NaOH used is 26.79 mL. The concentration of NaOH that get from the experiment is                 

which is nearly to

References:

1. O Level Chemistry – Chemical Definition of ALKALIS.(2011).Retrieved 21 June , 2012,from

2. (2008).Titration to Determine the Molarity of an Acid or Base. Retrieved 21 June, 2012, from 

3. Bishop.M.(2010).Acid-Base Tititration. Retrieved 21 June, 2012, from http://preparatorychemistry.com/Bishop_Titration.htm