Choosing apparatus, reading scales, anomalies, means, uncertainty, errors and the words for good data.
What you will learn
- Choose apparatus by its resolution
- Calculate a mean and its uncertainty
- Tell random from systematic errors
Good readings
- Resolution: the smallest change an instrument shows. A burette reads to 0.05 cm³; a measuring cylinder to 1 cm³ or worse.
- Read volumes at eye level from the bottom of the meniscus: the curved surface of a liquid in a narrow tube.
- Repeat, spot anomalies, leave them out, take a mean.
uncertainty ≈ ± half the range of the repeats
The percentage uncertainty compares the uncertainty with the reading: percentage uncertainty = uncertainty ÷ reading × 100. A burette reading of 20.00 cm³ with an uncertainty of ± 0.05 cm³ is 0.05 ÷ 20.00 × 100 = 0.25%; the same volume in a measuring cylinder (± 1 cm³) is 5%, so the burette is the better choice.
The rest of this lesson and its 8 practice questions are locked.
In this lesson
- Good readings
- Errors and the words for data
8 practice questions, marked as you go; about 20 minutes in all.
Key terms
Anomalies, Mean, Percentage uncertainty, Random errors, Systematic errors
In this unit: Required practicals and maths skills
The skills behind every chemistry practical and calculation: units, uncertainty, titrations, graphs, the equations and planning.