The purpose of this lab is to use volumetric analysis to determine the concentration of unknown substances. A sodium hydroxide solution is standardized to assist in finding the concentration of an acetic acid. An indicator must be used to pin point the equivalence point, the point in which 1 mole of a substance is equal to 1 mole of another. When that is found, we can determine the concentration.
HC2H3O2 (aq) + NaOH (aq) H2O (l) + NaC2H3O2 (aq)
The above equation is used to neutralize the acetic acid. The acid reacts with a base to produce water and a salt. Because there’s a 1:1 ratio, the moles of the acid must equal the moles of the base in order to reach the equivalence point. As far as the indicators go, an acid-base indicator will be used to show when we are close to the end point. For example, when HIn is dissociated In - is produced and it is pink. (See equation below)
HIn + H2O H3O + +In –
Standardization of NaOH Solution
1. A known amount of KHP is transferred to an Erlenmeyer flask and an accurately measured amount of water is added to make up a solution.
2. NaOH solution is carefully added to the KHP solution from a buret until we reach the equivalence point. At the equivalence point, all the KHP present has been neutralized by the added NaOH and the solution is still colorless. However, if we add just one more drop of NaOH solution from the buret, the solution will immediately turn pink because the solution is now basic.
Titration of an unknown
1. A measured amount of an acid of unknown concentration is added to a flask using a buret. An appropriate indicator such as phenolphthalein is added to the solution. (The indicator will indicate, by a color change, when the acid and base has been neutralized).
2. Base (standard solution) is slowly added to the acid.
3. The process is continued until the indicator shows that neutralization has occurred. This is called the END POINT. The end point is usually signaled by a sharp change in the color of the indicator in the acid solution. In acid-base titrations, indicators are substances that have distinct different colors in acid and base (Phenolphthalein pink in base, colorless in acid).
4. At the equivalence point, both acid and base have been completely neutralized and the solution is still colorless. However, if we add just one more drop of NaOH solution from the buret, the solution will immediately turn pink because the solution is now basic. This slight excess of NaOH is not much beyond the end point. The volume of the base is recorded and used to determine the molarity of the acetic acid solution.
Standardization of NaOH solution
| Trial 1| Trial 2| Trial 3|
Mass of KHP| 0.297 g| 0.325 g| 0.309 g|
Initial buret reading, NaOH| 0.00 mL| 0.50 mL| 7.70 mL| Final buret reading, NaOH| 32.0 mL| 34.0 mL| 38.7 mL|
Volume used, NaOH| 32.0 mL| 33.5 mL| 31.0 mL|
Molarity of NaOH solution| 0.0454 M| 0.0475 M| 0.0488 M| Average molarity of NaOH| 0.0472 M|
Titration of unknown
| Trial 1| Trial 2| Trial 3|
Initial buret reading, NaOH| 2.70 mL| 19.9 mL| 0.00 mL| Final buret reading, NaOH| 19.9 mL| 36.2 mL| 19.8 mL|
Volume used, NaOH| 17.2 mL| 16.3 mL| 19.8 mL|
Molarity of acetic acid solution| 0.0780 M| 0.0769 M| 0.0935 M| Average molarity of acetic acid solution| 0.0828 M|
The following calculations were used for each Trial, but only inputs for Trial 1 will be shown below. Volume = Final buret reading – Initial buret reading
i. Volume of NaOH = Final buret reading of NaOH – Initial buret reading of NaOH ii. Volume of NaOH = 32.0 mL NaOH – 0.00 mL NaOH
iii. Volume of NaOH = 32.0 mL
Molarity = Moles/Liters
i. Molarity of NaOH solution = (mass of KHP/molar mass of KHP) / Volume of NaOH ii. Molarity of...