5.4 [Mistakes, errors, Accuracy and precision. Measurement uncertainty Percentage measurement uncertainty] Flashcards
What are errors?
Errors are the difference between an experimental value and the accepted or correct value.
What are mistakes?
A mistake is something that a skilled operator can avoid by being careful
What are some examples of mistakes?
Chemist weighs a beaker on a balance without making sure balance is set to 0. reading on balance is very different.
A student sees a burette reading as 27.35 and writes down 23.75.
What is the difference between accuracy and precision?
Accuracy is a measure of how close the results are to the true value.
Precision is a measure of how close the values are to each other.
What is a random error?
This is an error cause by an unpredictable change in conditions such as temperature or pressure, or by a difference in recording that is difficult to get exactly right.
Example would be mass of object measured using same balance but at different times during the day, values can differ as a result of change in temp or condensation.
What is a systematic error?
Error caused by the apparatus leads to the recorded value being too high or too low.
A 25cm3 pipette been wrongly calibrated during its manufacture so its graduation mark is lower than should be no matter how carefully used volume added always be less than 25cm3
What is measurement uncertainty?
When using apparatus there is always a potential error known as measurement uncertainty. Size of uncertainty is determined by precision of the apparatus.
What is the precision like for balances?
Digital balances have varying degrees of precision. A balance measuring to 3 decimal places is more precise than a balance that measures to 1. Meaning that the measurement uncertainty involved is lower for 3 decimal places.
What is the measurement uncertainty for different pieces of apparatus?
Burette: Capacity: 50cm3 MU: 0.05 (burette read 2 times though so measurement uncertainty in titration is 0.10cm3)
Pipette: Capacity: 25cm3 MU: 0.06CM3
Volumetric flask: Capacity: 250cm3 MU: 0.3cm3
All are plus or minus Measurement uncertainty
What is percentage uncertainty?
Percentage uncertainty in an experiment is Actual measurement uncertainty x 100 / divided by the value recorded.
