Topic 11: Measurement and Data Processing

Question 1

Random Errors

Definition with examples.

Answer 1

• are caused by unpredictable changes in the experiment (in the conditions or apparatus)
• equal probability of the measured alue being too high or too low
• examples:
  
  • insufficent data
  • misinterpreting the reading
  • changes in the enironment during the experiment

Question 2

Ways random errors can be fixed:

Answer 2

• cannot be eliminated but can be reduced by conducting repeat trials
• can be reduced by using precise apparatus

Question 3

Systematic Errors

Definition with examples.

Answer 3

• occur as a result of a flaw in the experimental design of apparatus
• cause the measured value to be consistently higher or lower than the actual value
• cannot be reduced by conducting repeat trials
• examples:
  
  • losing a product (such as gas) in a reaction
  • forgetting to zero a mass balance
  • overshooting the endpoint in a titration
  • reading from the top of the meniscus when measuring volume

Question 4

Percentage Error

Definition and Formula.

Answer 4

• measure of how close the experimental value is to the theoretical or accepted value
• Percentage Error = [(Experimental value-Theoretical value) /Thoretical value] × 100%

Question 5

Absolute Uncertainty

Definition.

Answer 5

• The absolute uncertainty of digital apparatus is ± the smallest scale division
• Absolute uncertainty of analog apparatus is ± half the smallest scale division

Question 6

Percentage Uncertainty

Definition.

Answer 6

• Percentage Uncertainty = (Absolute uncertainty/ Measurement) × 100%

Question 7

Index of Hydrogen Deficiency (IHD)

Definition and Formula.

Answer 7

• is a count of how many molecules of H2 need to be added to convert the molecule to the corresponding, saturated, non-cyclic molecule.
• a degree of unsaturation for a hydrocarbon with x carbon atoms and y hydrogen atoms: IHD = (0.5)(2c + 2 – h – x + n)

Question 8

IHD for
a) single bonds
b) double bond/ ring structure
c) triple bond

Answer 8

a) 0
b) 1
c) 2

Question 9

What are three types of spectroscopy?

Answer 9

1. Infrared Spectroscopy
2. HNMR Spectroscopy
3. Mass Spectroscopy

Question 10

Infrared Spectroscopy

Answer 10

• the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light.

Question 11

What type of bond interacts with IR radiation?

Answer 11

• polar covalent bond (non-polar bonds do not absorb IR radiation)
• The intensity depends on the dipole moment of the bond:
  
  • Strongly polar bonds produce strong bands
  • Bonds with medium polarity produce medium bands
• IR is not generally used to determine the whole structure of an unknown molecule

Question 12

What does an infrared spectroscopy show?

Answer 12

• When molecules absorb energy in the IR region of the electromagnetic spectrum, it causes the bonds between the atoms to vibrate (the bonds stretch and bend)
• The frequency of IR radiation that is absorbed is measured as the number of waves per centimeter
• The fingerprint region can be used to identity an unknown compound by comparing with the IR spectra of known compounds

Question 13

How to analyze an IR graph?

Answer 13

• Tongue: A broad, rounded peak in the region
• Swords: These peaks are almost always the strongest peaks in the entire spectrum and are relatively narrow, giving them a somewhat “sword-like” appearance
• Once we have identified our “tongue” and our “swords” compare the wavelength value to the value in Table 26
• the graph can be divided up into a functional group region, and the fingerprint region. The fingerprint region is located on the right while the functional group region is located on the left and involves the tongue

Question 14

Mass Spectrometry

Answer 14

• is used to determine the relative atomic mass (Ar) of an element it can **also be used to determine the structure of a compound **

Question 15

What does a Mass spectrometry show?

Answer 15

• Inside the mass spectrometer, some of the molecular ions break down to produce fragments
• A fragmentation pattern is produced which gives useful information about the structure of the compound
• When determining the structure of a compound don’t use the mass/charge (m/z) value
• Instead take the largest value and subtract every other value from it

Question 16

1H NMR: Proton nuclear magnetic resonance spectroscopy

Answer 16

• technique used to determine a compound’s unique structure

Question 17

How to analyze an 1H NMR: Proton nuclear magnetic resonance spectroscopy graph?

Answer 17

• The position of the NMR signal is measured relative to the signal produced by TMS tetramethylsilane
• The chemical shift is measured relative to the point 0
• If there are two of the same group (two CH3 groups), look at the groups of atoms that those groups are bonded to, if they are the same then the protons are in the same chemical environment, if not they are in different chemical environments
• First look at:
  
  • Number of hydrogen types
  • Peaks
  • Neighbors
  • Cause of shift