Measurement and Data Processing

Question 1

How do you measure the uncertainty in analogue instruments?

Answer 1

+/- half the smallest division

Question 2

how do you measure the uncertainty in digital instruments?

Answer 2

+/- half the smallest division

Question 3

What are other sources of uncertainty?

Answer 3

reaction time

judging what is the

• equivalence point
• temperature at a particular time
• voltage of a cell

Question 4

significant figures

Answer 4

digits in the measurement up to and including the first uncertain digit

Question 5

experimental error

Answer 5

difference between the recorded value and the generally excepted value

Question 6

what are the two categories of error?

Answer 6

random or systematic

Question 7

random error

Answer 7

equal probability of result being too high or too low

Question 8

what causes random error?

Answer 8

• readability of measuring instrument
• effects of changes in surroundings (temperature variations, air currents)
• insufficient data
• misinterpretation of data

Question 9

how can random error be reduced?

Answer 9

repeated measurements

Question 10

repeatable

Answer 10

the same person duplicates the experiment with the same results

Question 11

reproducible

Answer 11

several experimenters duplicate the results

Question 12

systematic error

Answer 12

• poor experimental design or procedure
• cannot be reduced by repeating the experiment

Question 13

what causes systematic error?

Answer 13

• measuring volume from top of the meniscus
• overshooting volume in titration
• heat losses in exothermic reaction

Question 14

uncertainty when add or subtract measurements?

Answer 14

sum of absolute uncertainties

Question 15

uncertainty when multiply or divide measurements

Answer 15

sum of individual percentage uncertainties

Question 16

precision of data when multiply or divide data

Answer 16

same number of significant figures at the least precise datum

Question 17

precision of data when add or subtract data

Answer 17

same number of decimal places as the least precise value

Question 18

qualitative analysis

Answer 18

detects presence but not quantity of substance in a mixture e.g. chromatography

Question 19

quantitative analysis

Answer 19

measurement of the quantity of a particular substance in a mixture

Question 20

structural analysis

Answer 20

• description of how the atoms are arranged in molecular structures
• e.g. spectroscopy

Question 21

infrared spectroscopy

Answer 21

identify bonds in a molecule

Question 22

mass spectrometry

Answer 22

determines relative atomic and molecular masses

Question 23

nuclear magnetic resonance spectroscopy

Answer 23

shows chemical environment of certain isotopes

Question 24

IHD

Answer 24

• 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

Question 25

equation for IHD

Answer 25

(2C + 2 - H)/2

Question 26

double bond IHD

Answer 26

1 IHD

Question 27

ring IHD

Answer 27

1 IHD

Question 28

triple bond IHD

Answer 28

2 IHD

Question 29

do O or S affect IHD?

Answer 29

no

Question 30

what are halogens counted as in IHD?

Answer 30

like H atoms

Question 31

N IHD

Answer 31

add one to the number of C and one to the number of H

Question 32

what happens in the mass spectrometer?

Answer 32

* 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

Question 33

what happens in the infrared spectroscope?

Answer 33

* 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

Question 34

what happens in a proton nuclear magnetic resonance spectroscope?

Answer 34

* 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