Topic 11: Measurement and data processing Flashcards
Quantitative data
Data taken from measurements made in the laboratory and is associated with random errors
Qualitative
Non-numerical data obtained from observations, not from measurements.
Uncertainty of digital apparatus
The absolute uncertainty of digital apparatus is ± the smallest scale division.
For example a mass balance that can measure up to two decimal places has an absolute uncertainty of ± 0.01 g
Percentage uncertainty is the absolute uncertainty divided by the measured value. multiplied by 100 (gives a percentage)
Uncertainty of analog apparatus
The absolute uncertainty of analog apparatus is ± half the smallest scale division
For example a measuring cylinder with a smallest scale division of 1cm^3 has an absolute uncertainty of ± 0.5 cm^3
To calculate percentage uncertainty we divide the absolute uncertainty by the measured value, and then multiple by 100 (gives a percentage)
Uncertainty of a burette
A burette measures to two decimal places with the last digit being a 0 or a 5
The absolute uncertainty of a burette is ± 0.05cm^3
To calculate percentage uncertainty, divide the absolute uncertainty by the measured value, then multiple by 100 (gives a percentage)
Random errors
Give an example.
How can they be prevented?
Random errors are caused by unpredictable changes in the experiment (in the conditions or apparatus).
With random errors, there is an equal probability of the measured value being too high or too low.
An example of a random error:
- A change in the environment during the experiment (such as a change in the room temperature).
- Observer misinterpreting the reading.
- Insufficient data (not conducting repeat trials).
Random errors cannot be eliminated but can be reduced by conducting repeat trials.
They can also be reduced by using precise apparatus (such as using a volumetric pipette rather than a breaker to measure volume).
Systematic errors
Give an example.
Systematic errors occur as a result of a flaw in the experimental design or apparatus.
Systematic errors cause the measured values to be consistently higher or lower than the actual value. They cannot be reduced by conducting repeat trials.
Examples of systematic errors:
- Heat loss in an experiment to measure enthalpy change.
- Losing a product (such as a gas) in a reaction.
- Overshooting the endpoint in a titration.
- Reading from the top of the meniscus when measuring volume.
- Forgetting to zero a mass balance
How to propagate uncertainty
Calculate percentage uncertainty and add them together.
Percentage error
A measure of how close the experimental value is to the theoretical value
(experimental value) — (theoretical value) % error = ——————————————————— x 100
(theoretical value)
Index of hydrogen deficiency (IHD)
A count of how many molecules of hydrogen need to be added to convert the molecule to the corresponding saturated, non-cyclic molecule
Rules for calculating IHD
- Sulfur and oxygen do not affect the IHD
- Halogens are treated like H atoms
- For each N atom, add one to the number of C atoms and H atoms
What do the peaks in a H NMR spectrum show?
The number of peaks gives the number of different chemical environments in which hydrogen atoms are located
TMS
The chemical shift is measured relative to TMS (tetramethylsilane)
What does the size of each peak in a H NMR spectrum represent
The peaks are proportional to the number of hydrogen atoms in the chemical environment
Infrared spectroscopy
When molecules absorb energy in the infrared region of the electromagnetic spectrum, it causes the bonds between the atoms to vibrate.
The frequency of IR radiation that is absorbed is measured as the number of waves per centimetre (wave number)
