3.1.4Quantitative Chemistry
Chemical Measurements — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 10
Section 1 of 6Types of error
| Feature | Random error | Systematic error |
|---|---|---|
| Effect on results | Scatter above and below the true value | All results are shifted the same way, always too high or always too low |
| Causes | Reading a scale, changes in lab conditions such as temperature or draughts | Faulty equipment or poor method |
| Example | Estimating the last digit on a thermometer | Forgetting to zero a balance, so every mass is too high |
| Reduced by | Repeating the measurement and calculating a mean | Improving the method or calibrating the equipment; repeating does not help |
Section 2 of 6Other examples of systematic error
Zero error
- Zero error A balance that is not zeroed adds the same extra mass to every reading.
Reading position
- Reading position Reading a burette from above or below eye level gives volumes that are consistently wrong.
Heat loss or evaporation
- Heat loss or evaporation In a calorimetry experiment, an uncapped spirit burner loses fuel by evaporation, so the mass lost is too large.
Section 3 of 6Calculating uncertainty
Uncertainty of a measurement
Analogue instruments
- Analogue instruments Uncertainty is ± half the smallest division on the scale.
- A thermometer marked in 1 °C has an uncertainty of ±0.5 °C.
Digital instruments
- Digital instruments Uncertainty is ± the smallest reading the display shows.
- A balance reading to 0.01 g has an uncertainty of ±0.01 g.
Repeated results
- Repeated results Uncertainty of the mean is ± half the range of the results.
Judgement
- Judgement Some uncertainties come from deciding when something changes, such as the end-point colour of an indicator.
Section 4 of 6Uncertainty from repeats
1
Record each repeat result and calculate the mean.
2
Find the range by subtracting the smallest value from the largest.
3
Divide the range by two to find the uncertainty.
4
Write the answer as mean ± uncertainty. For titres of 24.2, 24.4 and 24.0 cm3, the mean is 24.2, the range is 0.4 and the uncertainty is ±0.2 cm3.
5
A smaller uncertainty indicates results that are closer together, which suggests greater precision.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Random error | An unpredictable error that makes readings scatter around the true value. |
| Systematic error | An error caused by faulty method or equipment that shifts every result in the same direction. |
| Uncertainty | The range of values within which the true value is likely to lie. |
| Mean | The sum of the results divided by the number of results. |
| Range | The difference between the largest and smallest values in a data set. |
| Precision | How close repeated measurements are to each other. |
Section 6 of 6Exam technique
1
Saying repeating readings removes systematic errors. It does not; the same fault shifts every result.
2
Using the full smallest division as the uncertainty for an analogue instrument. It is half.
3
Dividing the range by the number of results instead of by two.
4
Mixing up the effectsrandom errors scatter results, systematic errors shift them in one direction.
5
Leaving out the unit with the uncertainty value.
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