Measurement and Data Processing Flashcards
How do you measure the uncertainty in analogue instruments?
+/- half the smallest division
how do you measure the uncertainty in digital instruments?
+/- half the smallest division
What are other sources of uncertainty?
reaction time
judging what is the
- equivalence point
- temperature at a particular time
- voltage of a cell
significant figures
digits in the measurement up to and including the first uncertain digit
experimental error
difference between the recorded value and the generally excepted value
what are the two categories of error?
random or systematic
random error
equal probability of result being too high or too low
what causes random error?
- readability of measuring instrument
- effects of changes in surroundings (temperature variations, air currents)
- insufficient data
- misinterpretation of data
how can random error be reduced?
repeated measurements
repeatable
the same person duplicates the experiment with the same results
reproducible
several experimenters duplicate the results
systematic error
- poor experimental design or procedure
- cannot be reduced by repeating the experiment
what causes systematic error?
- measuring volume from top of the meniscus
- overshooting volume in titration
- heat losses in exothermic reaction
uncertainty when add or subtract measurements?
sum of absolute uncertainties
uncertainty when multiply or divide measurements
sum of individual percentage uncertainties
precision of data when multiply or divide data
same number of significant figures at the least precise datum
precision of data when add or subtract data
same number of decimal places as the least precise value
qualitative analysis
detects presence but not quantity of substance in a mixture e.g. chromatography
quantitative analysis
measurement of the quantity of a particular substance in a mixture
structural analysis
- description of how the atoms are arranged in molecular structures
- e.g. spectroscopy
infrared spectroscopy
identify bonds in a molecule
mass spectrometry
determines relative atomic and molecular masses
nuclear magnetic resonance spectroscopy
shows chemical environment of certain isotopes
IHD
- index of hydrogen deficiency
- measure of how many molecules of H2 would be needed in theory to convert the molecule to the corresponding saturated, non-cyclic molecule
equation for IHD
(2C + 2 - H)/2
double bond IHD
1 IHD
ring IHD
1 IHD
triple bond IHD
2 IHD
do O or S affect IHD?
no
what are halogens counted as in IHD?
like H atoms
N IHD
add one to the number of C and one to the number of H
what happens in the mass spectrometer?
- vaporized sample becomes ionized to form cation
- some of molecular ions break down to give fragments - also deflected by the external magnetic field
- show up as peaks on detector
- identify particular fragments by looking at difference in mass between the parent peak and fragments
what happens in the infrared spectroscope?
- when molecules absorb energy in the infrared region they vibrate (bonds stretch and bend)
- energy absorbed depends on particular bond and other groups attached to the two atoms forming the bond
- possesses a ‘fingerprint’ region: characteristic pattern between about 1400-400cm-1 which is specific to a particular compound
- possible to identify an unknown sample by comparing fingerprint region to library of known compounds
what happens in a proton nuclear magnetic resonance spectroscope?
- 1H NMR provides information on chemical environment of all hydrogen atoms in the molecule
- nuclei of hydrogen atoms possess spin (can exist in two possible states of energy)
- if strong magnetic field applied spin either align themselves with or against magnetic field
- small energy difference between these states - nuclei can absorb a photon of energy when going from higher to lower spin state
- photon energy small and in radio region
- chemical shift: position in 1H NMR spectrum where absorption occurs for each hydrogen atom
- height of each section is proportional to number of hydrogen atoms in each chemical environment
