Chapter 11: Measurements and data processing Flashcards
What are random uncertainties causes by?
Random uncertainties are caused by the limitations of the measuring apparatus and other uncontrollable variables that are inevitable in any experiment.
What are several sources of random uncertainties?
Variations due to air currents in the room, the heating effects of the current in the circuits, friction between various mechanical parts etc
How can the effect of random uncertainties be reduced?
Repeating the measurements more often
They can never be completely eliminated
How to calculate the uncertainty of a measurement?
It is half the smallest division to which you take a reading.
for example,
on the 100cm3 measuring cylinder, it is divided apart by 1cm3 and so the uncertainty is + or - 0.5cm3
On an electronic balance, it reads to two decimal places, the smallest division is 0.01g, what is the uncertainty?
±0.005 g
What does precision relate to?
Precision relates to the reproducibility of results. If a series of readings is taken with high precision, it indicates that the repeated values are all very close together and close to the mean (average) value.
What does accuracy refer to?
Accuracy refers to how close a measurement is to the actual value of a particular quantity.
What causes systematic errors and can it be reduced?
A systematic error is an error introduced into an experiment by the apparatus or the procedure.
It cannot be reduced by repeating the readings?
What are some example of systematic errors?
- the beaker is not that well insulated so heat will escape – the measured temperature rise will be less than the actual value
- the reaction does not occur instantaneously and the thermometer does not respond instantaneously and so the measured temperature rise will be less than the actual value
- the concentration of the sodium hydroxide is less than 0.10moldm−3 and so the measured temperature rise will be less than the actual value
How to calculate percentage error?
What is the difference between accuracy and precision?
Precision: the reproducibility of results
Accuracy: how close a value is to the true value of the measurement
What does it mean when the percentage error is greater than the percentage random uncertainty?
this suggests that the experiment involves some systematic errors
How can systematic errors be reduced?
By changing the way the experiment is carried out
See this example, how do we find out the uncertainty of the temperature change?
Add 0.02 +0.02 together
Wen quantities with uncertainties are added or subtracted, what is done to the absolute uncertainty?
They are added
When multiplying or dividing quantities with uncertainties, what is done to the uncertainties?
The percentage uncertainties should be added
What is done to the percentage uncertainty when multiplying or dividing a quantity with a pure number?
The absolute uncertainty is multiplied/divided by that number so that the percentage uncertainty stays the same
How should the uncertainty be quoted?
The uncertainty is usually quoted to one sig fig and your measurement should be stated so that the uncertainty is in the last sig dig - no figures should be quoted after the uncertainty
What does the gradient represent in a graph?
The rate of the reaction
What is an infrared spectroscopy used for?
To determine the structure of an organic compound
Electromagnetic radiation in the range 400-4000cm01 is passed through a sample. The printout of the spectrum then shows which frequencies (wavenumber) are absorbed. Can be used to determine which bonds are present in a molecule because the precise wavenumber at which infrared radiation is absorbed by a particular functional group depends on the adjacent atoms.
What happens to a sample of organic compound is injected into a mass spectrometer?
- Bombarded with high energy electrons to produce positive ions.
- Pass on through the mass spectrometer where they are separated according to mass and detected
What are the other peaks other than the molecular ion peak?
When do fragmentation pattern arise?
When a molecule can break apart into smaller fragments when it is bombarded by high energy electrons
Only what type of ions give peaks in a mass spectrum?
Positive ions
What elements nucleus has a property called spin?
Hydrogen