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Calculate percentage uncertainty in titrations - A level Chemistry (OCR AQA Edexcel) not so easy ;)
Ask Fran Chemistry
Overview
This video explains how to calculate percentage uncertainty in titration experiments, focusing on common pitfalls. It details how to correctly calculate the titre volume by subtracting initial from final readings, emphasizing the need for two decimal places in the answer. The video then explains how to calculate the mean titre using only concordant results and finally, how to determine the percentage uncertainty, highlighting that the uncertainty from each reading must be doubled because two measurements are taken.
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Chapters
- Titre volume is calculated by subtracting the initial burette reading from the final burette reading.
- Results must be recorded to two decimal places to maintain precision.
- An answer like 24.0 is insufficient; 24.00 is required.
Accurate calculation of titre volume and maintaining the correct number of decimal places are crucial for the precision of experimental results in titrations.
If the final reading is 24.05 cm³ and the initial reading is 1.05 cm³, the titre is 23.00 cm³.
- The mean titre should only be calculated using concordant results.
- Concordant results are those that are close to each other, typically within 0.1 cm³.
- The average of these concordant results gives the mean titre.
Using only concordant results ensures that the mean titre is representative of reliable measurements, leading to more accurate subsequent calculations.
If titres are 23.45 cm³, 23.55 cm³, and 24.05 cm³, only the first two (23.45 and 23.55) are concordant and should be used to calculate the mean.
- Percentage uncertainty is defined as (uncertainty / measured value) * 100.
- In titrations, the uncertainty of each individual reading (e.g., from the burette) must be considered.
- Since two readings are taken (initial and final), the total uncertainty is doubled (multiplied by 2).
- The final percentage uncertainty must be reported to two decimal places.
Understanding how to correctly calculate percentage uncertainty accounts for the inherent limitations in measurement precision, providing a realistic estimate of the reliability of the experimental results.
If the uncertainty for one burette reading is ±0.05 cm³ and the titre is 23.40 cm³, the percentage uncertainty is ((0.05 * 2) / 23.40) * 100, which should be rounded to two decimal places.
Key takeaways
- Titration results require a precision of two decimal places, even if the calculation results in trailing zeros.
- Only concordant titre volumes (typically within 0.1 cm³ of each other) should be used to calculate the mean titre.
- The uncertainty in a titration titre is doubled because it is derived from two separate measurements (initial and final burette readings).
- Percentage uncertainty quantifies the reliability of a measurement by comparing the uncertainty to the measured value.
- Accurate calculation of percentage uncertainty is essential for evaluating the quality of experimental data in quantitative chemistry.
Key terms
TitreConcordant resultsMean titrePercentage uncertaintyInitial readingFinal readingUncertainty
Test your understanding
- How is the titre volume calculated from burette readings, and why is precision important?
- What criteria define concordant results, and why are they essential for calculating the mean titre?
- Why is the uncertainty in a titration typically doubled when calculating percentage uncertainty?
- What is the formula for percentage uncertainty, and how does it apply to titration data?