A. Using the average molarity of NaOH calculate % error in your determination of the molarity of NaOH if the true value of the molar concentration were 0.1021 M. The Molarity of NaOH is 1.057 M
B. If your experimental molar concentration of NaOH were not close to the accepted value what are two likely reasons for the discrepancy. Do not simply say “human error.” Explain clearly and specify whether the cause of error would lead to a higher or lower experimental molar concentration of NaOH.
C. A beret is graduated with zero at the top and 50.00 mL at the bottom. If the beret is filled with water and has an initial beret reading of 1.08 mL and 35.28 mL is removed from the tip, what would the final beret reading be?
D. The purpose of the experiment (in Part I) was to determine the concentration of a NaOH solution. During the titration more water was added to wash down the sides of the flask. This would change the concentration in the flask. Why is this not a concern? Explain your answer.
The Correct Answer and Explanation is :
A. Calculation of % Error
To calculate the percentage error in the determination of the molarity of NaOH, we use the following formula:
[
\text{Percentage Error} = \left| \frac{\text{Experimental Value} – \text{True Value}}{\text{True Value}} \right| \times 100
]
Given:
- Experimental Molarity of NaOH = 1.057 M
- True Molarity of NaOH = 0.1021 M
Substituting the values into the formula:
[
\text{Percentage Error} = \left| \frac{1.057 – 0.1021}{0.1021} \right| \times 100 = \left| \frac{0.9549}{0.1021} \right| \times 100 = 935.51\%
]
So, the % error in the determination of the NaOH molarity is 935.51%.
B. Likely Reasons for Discrepancy
If your experimental molar concentration of NaOH is not close to the accepted value, two potential reasons for the discrepancy could be:
- Contamination of NaOH Solution:
If the NaOH solution became contaminated, such as by absorbing CO₂ from the air, this would cause the concentration of NaOH to decrease because CO₂ reacts with NaOH to form Na₂CO₃. This would result in a lower experimental molarity than the accepted value because less NaOH would be available for titration. - Inaccurate Measurement of Volume of NaOH:
If the volume of NaOH was measured inaccurately, for example, by not reading the burette correctly or failing to note the meniscus at eye level, this could lead to an overestimation or underestimation of the volume of NaOH used. If too much NaOH were added, the calculated molarity could be lower because the actual volume of NaOH would be smaller than recorded, and vice versa if too little NaOH was used, resulting in a higher experimental molarity.
C. Final Beret Reading Calculation
The final beret reading can be calculated by subtracting the volume of water removed from the initial volume. Given:
- Initial beret reading = 1.08 mL
- Volume of water removed = 35.28 mL
To find the final reading, subtract the volume removed:
[
\text{Final Reading} = 1.08 \, \text{mL} – 35.28 \, \text{mL} = -34.2 \, \text{mL}
]
However, a negative value is nonsensical in this context, as volume cannot be negative. This suggests there may have been a misunderstanding in the interpretation of the beret’s scale or an error in the volume measurement.
D. Why Adding Water During Titration is Not a Concern
When performing a titration, additional water may be added to the flask to wash down any NaOH that adheres to the sides of the flask, ensuring that all the titrant reacts with the analyte. However, this is generally not a concern for the following reasons:
- Water is a Non-reactive Solvent: The water added to the flask does not affect the stoichiometry of the titration because it does not participate in the chemical reaction between NaOH and the analyte. It simply dilutes the solution temporarily.
- Constant Concentration of NaOH in the Burette: The concentration of NaOH in the burette remains constant throughout the titration, even if water is added to the flask. The purpose of the titration is to determine the amount of NaOH needed to neutralize the analyte. Adding water will not affect the volume of NaOH used in the titration, so the titration still proceeds with the same volume of NaOH required for the reaction.
In summary, while the water added during the titration can dilute the NaOH in the flask momentarily, it does not affect the overall calculation of the NaOH concentration, as the volume used in the titration and the reaction’s stoichiometry remain unaffected.