Chemistry uses titration reactions to find the concentration of unknown solutions. A titration reaction involves reacting a base and acid such that the concentration of one solution is known (ChemCollective, 2021). The mole-to-mole ratio is then used to find the molarity of the unknown solution. If the concentration of a basic solution is known, the mole to mole ratio can be used to establish the concentration of an acid which is the number of moles of ions in a solution. The concentration of base refers to the concentration of ions in a solution. The experiment involves reacting sodium hydroxide (NaOH) of unknown concentration and the 0.500 M potassium hydrogen phthalate (KHP) standard solution to determine the molarity of sodium hydroxide.
Procedure
1. Load 0.500 M potassium hydrogen phthalate (KHP) solution onto the workbench.
2. Record the volume, pH, and concentration as they appear on the information window located on the left.
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3. Add 0.5 ml of phenolphthalein indicator into the 0.5 M KHP solution using a dropper
4. Load NaOH to the workbench and add 50 mL into the burette.
5. Fill the burette by gradually adding small amounts up to the zero mark
6. Record the volume of NaOH as indicated in the information window located on the left-hand side.
7. Gradually add NaOH into the solution of KHP from the burette while observing the pH and any color change.
8. Stop when the pH is 7, and the indicator turns pink.
9. Record the final volume of NaOH remaining in the burette.
Data Analysis and Calculations
| Trial 1 | Trial 2 | Trial 3 | Average | |
| Volume of KHP (mL) |
25 |
25 |
25 |
25 |
| Concentration of KHP (M) |
0.5 |
0.5 |
0.5 |
0.5 |
| Moles of KHP (mol) |
0.0125 |
0.0125 |
0.0125 |
0.0125 |
| Initial pH |
2.75 |
2.75 |
2.75 |
2.75 |
| Initial H+ concentration (M) |
0.00177302 |
0.001773 |
0.001773 |
0.001773 |
| Intitial OH- concentration (M) |
5.69E-12 |
5.69E-12 |
5.69E-12 |
5.69E-12 |
| Initial volume of NaOH (mL) |
52 |
52 |
52 |
52 |
| Final volume of NaOH (mL) |
38.53 |
38.53 |
38.53 |
38.53 |
| Final pH |
7 |
7 |
7 |
7 |
| Final H+ concentration (M) |
9.90E-08 |
9.90E-08 |
9.90E-08 |
9.9E-08 |
| Final OH- concentration (M) |
1.02E-07 |
1.02E-07 |
1.02E-07 |
1.02E-07 |
| Volume of NaOH used (Initial Volume -Final Volume) |
1.35E+01 |
1.35E+01 |
1.35E+01 |
13.47 |
| Moles of base (NaOH) (use initial moles of KHP to calculate) | 0.0125 | 0.0125 | 0.0125 | 0.0125 |
| Concentration of NaOH (M) | 0.928 | 0.928 | 0.928 | 0.928 |
Number of moles = Volume in L * molarity
Trial 1
Moles of KHP (mol) = 0.5 M * 25 mL = 0.0125 moles
Trial 2
Moles of KHP (mol) = 0.5 M * 25 mL = 0.0125 moles
Trial 3
Moles of KHP (mol) = 0.5 M * 25 mL = 0.0125 moles
Average = 0.0125 moles
Number of moles of NaOH
The mole ratio of the reaction of KHP: NaOH = 1:1 (ChemCollective, 2021).
For neutralization to take place, every 1 mole of KHP reacts with 1 mole of NaOH
The number of moles of the acid = number of moles of base.
The number of moles of NaOH used up in the reaction is = 0.0125 moles
Concentration of NaOH
Concentration = Number of moles/Volume in liters ( Soult, 2019).
The volume of NaOH used =initial volume - the final volume
Trial 1
Volume of NaOH used =initial volume - final volume = 1.35E+01 mL
Concentration = 0.0125 moles / ( 1.35E+01mL *1L/1000 mL) = 0.928 M
Trial 2
Volume of NaOH used =initial volume - final volume = 1.35E+01 mL
Concentration = 0.0125 moles / ( 1.35E+01mL *1L/1000 mL) = 0.928 M
Trial 3
Volume of NaOH used =initial volume - final volume = 1.35E+01 mL
Concentration = 0.0125 moles / ( 1.35E+01mL *1L/1000 mL) = 0.928 M
Average = 0.928 M
The pH of the KHP acid increase from 2.75 to 7 with the addition of the NaOH solution. In a reaction between an acid and base, the ions in the acid and ions in the base react to form water hence the decrease in acidity with each addition of NaOH. When the pH is 7, it means that all the excess hydroxonium ions and hydroxide ions have reacted to form water. The phenolphthalein indicator in the KHP solution is colorless, given that the solution is an acid. After the initial addition of phenolphthalein indicator into the KHP solution, no color change was observed. The addition NaOH into the KHP solution caused an increase in pH. However, the solution remained colorless and only changed to pink at a pH of 7. The phenolphthalein indicator remains colorless in a neutral solution and only turns pink when the solution first becomes basic.
A titration experiment involves reacting an acid and a base to find the concentration. Therefore, the concentration of one of the reactants is known while the other is unknown. The concentration of KHP acid used in the solution is 0.5 M. Therefore, the number of moles can be calculated as the product of molarity and volume used. Since 25 mL of the acid was used in the reaction, the number of moles can be estimated using the formula:
Number of moles = Volume in liters * Molarity ( Soult, 2019).
Number of moles = 25ml * 0.5 moles/liter = 0.0125 moles
The mole ratio of KHP acid to NaOH is 1:1. From the ratio, it means that 0.0125 moles of KHP acid would require an equal number of alkaline moles to neutralize. Neutralization means that the moles of acid react entirely with all the basic moles to form water. Consequently, the number of moles of NaOH used up in the reaction is 0.0125 moles. The volume of NaOH used to neutralize the KHP acid is the difference between the initial volume of NaOH in the burette and the final volume of NaOH that remains at the end of the reaction. Upon finding the volume of the NaOH used in the reaction, the concentration is calculated using the formula.
Molarity = number of moles/ Volume in liters ( Soult, 2019).
Volume of NaOH used =initial volume - final volume = 1.35E+01 mL
Concentration = 0.0125 moles / ( 1.35E+01mL *1L/1000 mL) = 0.928 M
Since there are 3 trials, the concentration of NaOH is equal to the average concentration for three trials.
Average = 0.928 M
Conclusion
The concentration of NaOH is 0.928 M . The calculations are based on the mole ratio of 1:1 for KHP and NaOH. A ratio of 1:1 means that the number of moles for KHP and NaOH used up in the neutralization reaction are equal. Concentration is calculated by dividing the number of moles by the volume in liters.
References
ChemCollective. (2021). Standardization of NaOH with a KHP Solution: Acid Base Titration . https://chemcollective.org/vlab/101
Soult, A. (2019, June 10). 8.1: Concentrations of solutions . Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Book%3A_Chemistry_for_Allied_Health_(Soult)/08%3A_Properties_of_Solutions/8.01%3A_Concentrations_of_Solutions#
